Online resources for all kinds of tabletop role playing systems researched by Sophie from Role Play Haven 

System-less resources

1shotrpg
Free adventures with various system formats, notably systems like D&D 5e, Cyberpunk, Warhammer, Call of Cthulhu, GURPS. 
https://1shotadventures.com/adventure-index/

Blogs e.g trilemma
Pre-written adventures not locked to any sytem. You set when and what your players roll, use resources from your chosen system to get enemy and monster stat blocks.
http://blog.trilemma.com/search/label/adventure

Free and purchase websites
From free/pwyc to luxury bundles: adventures, resources and supplements for lots of systems, written by enthusiatic players just like you.

DM’s Guild
https://www.dmsguild.com/

Drive thu rpg
https://preview.drivethrurpg.com/ 

Paper miniature maker
https://papermini.arkanatools.com/

D100 tables for fantasy A variety of features and objects (mostly fantasy)
https://blog.d4caltrops.com/

Random generators for all sorts of things
Shops, treasure, NPC’s (genres/systems: fantasy, space, Bladerunner, Pathfinder, D&D)
https://donjon.bin.sh/

50 free maps from Silver Compass beautifully drawn maps
https://www.patreon.com/posts/95323263

Random maps
City, villages, natural areas, dungeons 
https://watabou.itch.io/

Dungeon maps (with multi page printing options)
https://probabletrain.itch.io/dungeon-scrawl

Character description generator (and many more )
https://www.seventhsanctum.com/generate.php?Genname=generalperson

Playing online

Token maker https://thefatefulforce.com/battle-resources/token-creator/
(the bottom of the page has a set of free humanoid npc tokens too)

Battle maps https://thefatefulforce.com/battle-resources/battle-maps/ 

Online forums like reddit/discord can be a treasure trove of community created resources (some for free, some on patreons or private websites)

Pathfinder

2e character builders
1: https://pathbuilder2e.com/
2: https://www.dyslexic-charactersheets.com/build/pathfinder2 

Free adventures Paizo free RPG Day PDF’s
Pathfinder 1 (19 options)
https://paizo.com/store/pathfinder/adventures/standalone/freeRPGDay/firstEditionFreeRPG
Pathfinder 2e (4 options)
https://paizo.com/store/pathfinder/adventures/standalone/freeRPGDay/secondEditionFreeRPG

Dungeons & Dragons

Basic rules for D&D 5e download from Wizards of the Coast
https://media.wizards.com/2018/dnd/downloads/DnD_BasicRules_2018.pdf
or view on DnD Beyond https://www.dndbeyond.com/sources/basic-rules

Free online character builders

Dnd Beyond character creator (some limitations/paid options)
https://www.dndbeyond.com/characters

DMs vault (good if you are trying 'point buy' for the first time, mostly customisable, no sign up necessary)
https://www.dungeonmastersvault.com/

Fast character (fast, fun, randomised pregens to play)
https://fastcharacter.com/

5e Companion App (check your phones app store to see if it, or other options are available )
https://www.aidedd.org/dnd-builder/index.php (limited customisation)

Free D&D 5e adventures

PDFs adventures from Wizards of the Coast
Six faces of death (lvl 11-13): An alien being, dark omens, and vanishing ships send the adventurers to a mysterious island, newly appeared in the Sea of Swords. But can the characters uncover the mysteries of the changing Island in time to save Faerûn from a terror from another plane? https://media.wizards.com/2018/dnd/dragon/21/DRA21_sixfacesdeath.pdf

Elfhunt (lvl 5): In the Jungles of Chult lies Camp Highmore, a trading place in the heart of the wilderness. Here the fittest rule, but an elf has recently tipped the scale of power by stealing secret militaryinformation. A group of adventurers are hired to track her down and bring her back to Camp Highmore. (Time estimate 14hr-28hr)
https://media.wizards.com/2017/dnd/dragon/Elfhunt_11397805.pdf

Winters splendor (adjustable lvl 1–4): You’re invited to the Midwinter Gala in the City of Splendors! Among the jovial festivities, a sinister mystery waits to be unwrapped. A two-to-four-hour holiday adventure
https://media.wizards.com/2018/dnd/dragon/23/DRA23_WintersSplendor.pdf 

Ice road trackers (lvl 1): A trek toward Icewind Dale turns deadlywhen an avalanche blocks the heroes’ passage through the Spineof the World. Rescued by a menagerie of strange creatures, the heroes are asked to return the favor in exchange for answers about what’s happening in the cold and frozen North.
https://media.wizards.com/2020/dnd/downloads/iceroadadv1_unsurefooting.pdf

The Scroll Thief (lvl 2): A series of odd thefts have local heads ofvarious schools in scholars square concerened link here https://media.wizards.com/2014/downloads/dnd/DDEX16_TheScrollThief.pdf

Barber of Silver Moon (lvl 4–6) : People have been disappearing atnight in the city of Silverymoon. Some vanish entirely, leavingbehind whispered rumors of fiends or other evil creatures havingspirited them away. Others return strangely altered, with theirmemories of having been kidnapped wiped clean and their mindsstrangely dulled—and always with remarkable haircuts. https://media.wizards.com/2017/dnd/dragon/DRA12_barbersilverymoon_jbt.pdf

Northern Palace (any level): In the farthest northern reaches of icydarkness stands a fabulous palace—the grand home of amysterious tribe of gift-giving elves. At least, rumors speak of sucha place. Great riches and rewards supposedly emanate from the Northern Palace, so surely any party of adventurers will find it aworthy location to seek out.
https://media.wizards.com/2017/dnd/dragon/DRA17_northernpalace.pdf

Crypts at Kelemvor: This scenario sees the adventuring party given an important quest to collect a number of broken magical seals and to light a number of magical braziers, all of which once protected the crypts of Kelemvor. In so doing, they can help to quell a devastating uprising of undead
https://media.wizards.com/2018/dnd/dragon/18/DRA18_CryptsKelemvor.pdf

Gribbits Detective Agency (lvl1): the powerful underworld figure known as the Beggar King, has vanished. adventurers must get to the bottom of where his boss has gone, and why.
https://media.wizards.com/2018/dnd/dragon/18/DRA18_GribbitsAgency.pdf

2022 One Page Dungeon Contest Compendium (pay what you want) 
https://spielknights.gumroad.com/l/2022opdc

Free adventures at D&D Beyond

Frozen Sick (progress from lvl 1–3): a strange malady that turned anexplorer into an ice statue, and which has since shown up in another of thevillage’s residents. The characters investigate the mystery while contendingwith the forces of the Uttolots, and Shadycreek Run.  
https://www.dndbeyond.com/sources/wa/frozen-sick#StoryOverview 

The next few are free but account required

Lightning Keep (lvl 3 ): Adventurers set out to clear the lightning keep of monsters and activate its magical shield to protect townspeople  
https://www.dndbeyond.com/claim/source/lightning-keep 

Prisoner 13 (lvl 4): adventurers are hired to break into a prison to retrieve info and an object  
https://www.dndbeyond.com/claim/source/prisoner-13 

Star Forge (lvl 16): your choosen story hook requires adventurers to explore the legendary Star Forge.
https://www.dndbeyond.com/claim/source/giants-of-the-star-forge 

Call of Cthulhu

For Players: quick start rules, including creating investigators https://www.chaosium.com/content/FreePDFs/CoC/CHA23131%20Call%20of%20Cthulhu%207th%20Edition%20Quick-Start%20Rules.pdf

For Keepers: quick start guide
https://www.chaosium.com/cthulhu-quickstart/

Two free adventures

Lightless Beacon: on a ship heading for Rockport, Massachusetts, on Monday, April12th, 1926, the night of the new moon. Due to a malfunction at the lighthouse on Beacon Island, their ship founders on nearby rocks.
Dead Boarder: This is a murder investigation, which centers on the discovery of abody in a locked room. There are secrets contained in the room to be uncovered by the investigators.
https://www.chaosium.com/cthulhu-adventures/ 

Cult of Chaos GM Programme. Sign-up to run a couple of public games a year (RPH gming and Con GMing would count ) and get access to a selection of adventures for Chaosium systems (Call of Cthulhu x8, Runequest x4, Pendragon x1, 7th Sea x1) https://www.chaosium.com/organized-play/

Warhammer

Character creator: https://hammergen.net/ 

If Looks Could Kill: When the adventurers agree to help Rutger Reuter, a charismatic, young merchant from Ubersreik, little do they realise what’s in store.  https://cubicle7games.com/if-looks-could-kill-pdf

Cyberpunk adventures

Homecoming: A rogue drone causes mayhem in the Upper Marina, 4th and 3rd streetintersection. Lawmen calls in for any local backup. Of course, you wouldn’t go if cash isn’t involved. Multiple casualties and only two guys in blue holding on for dear life. The drone, attached by a cable and shouting repeatedly “Be careful! Please! Help!”, holds a machine gun and is popping lead to everything that moves. Another day in paradise. https://poluspr.com/2021/03/16/cyberpunk-red-homecoming/

Special delivery: A fixer in urgent need of a few edgerunners. A last minute announcement for an important job. "Go to apartment 17C. Collar got the dates for a simple trade. Be prepared for any booster ganger fools in the Combat Zone. You might get caught in a middle of a firefight, choomba."
https://poluspr.com/2021/02/19/cyberpunk-red-special-delivery/

Vampire the Masqerade 

Accounts required
Players character building: https://app.demiplane.com/nexus/vampire

Story: Auld Sanguine https://marketplace.roll20.net/browse/module/14024/auld-sanguine

5th edition Quick-start adventure https://preview.drivethrurpg.com/en/product/251678/The-Monsters-A-Vampire-the-Masquerade-5th-Edition-Quickstart