This is a collection of links to free and pay what you want adventures and sourcebooks. There are larger lists, like this one at Dragonsfoot, but this is the stuff I think is especially good.
Many of these have print versions for sale as well if you want to reward the creator.
A Promise of Vengeance Fulfilled by R. Lawrence Blake (Laybyrinth Lord, 5th-7th)
Fabled Curse of the Brigand Crypt by R. Lawrence Blake (Labyrinth Lord, 1st-3rd)
The Courtyard of Gerald the Red by R. Lawrence Blake (Labyrinth Lord, 1st-3rd)
The Lost Staves of Maurath by R. Lawrence Blake (Labyrinth Lord, 4th-6th)
The Maze of Nuromen by Justin Becker & Michasel Thomas (Blue Home, 1st)
Free Adventures in Spanish - These are for the Adventures in the East Mark RPG, which is basically a Mentzer basic (BEC) retroclone
Owl Con Modules - Created for a Houston gaming convention, these 3 modules for Labyrinth Lord are surprisingly long. If you like, they request you donate to a Houston food bank.
Labyrinths of Madness by Gavin Norman. Lovecraftian sourcebook of sorts
Narcosa - Psychedelic sourcebook.
Theorems and Thaumaturgy by Gavin Norman. Excellent magic sourcebook. Everyone should download this. Yes, even you.
The Warden by Philip Morris. A new character class
Lost Classes for OSRIC - Versions of the Monk, Barbarian, Cavalier, Bard, and many others.
Paratime Design by Tim Hartin has a huge section of free maps. I really love his style.
Brave The Labyrinth (okay, actually PWYW)
The Randomizer - Windows program that does random tables. Make your own, but includes a lot, include some from the 1e DMG
Medieval Fantasy City Generator by watabou - Yeah, there are a bunch of these, but I like this one.
Pay what you want stuff can be free, but the creator will take money if you feel it's worth it. You can try it free, then go back and buy it again for money the second time.
Pay What You Want Adventures
Baba Smerta - 5th-6th level adventure from QuasarDragon Games
Howler from J.V. West (comes in OSR and 3.x form in one package)
Knowledge Illuminates - Starter adventure
Proteus Sinking from Bjorn Warmedal
Sleeping Place of the Feathered Swine - Weird adventure
Taglar's Tomb - Not bad, but kinda short
The Beleagured Burrow - Short, cute gnome adventure
The Caverns of Ugard - Short low level adventure
The Caves of Moreau County - Modular adventure.
The Caves of Ortok from Infibadger Games
The Curse of Cragbridge - Low level Labyrinth Lord adventure
The Dungeon of Crows - Part One Part Two
The Lizardman of Ilzahatch from 3 Toadstools blog for 3rd level characters
The Lair of Largash the Lurid from Canister & Grape for beginner characters
The Temple of Asibare from QuasarDragon Games, 2-4th level
The Stolen Child from QuasarDragon Games. Highish level.
The Tomb of Rakoss the Undying from Mischief Games. Tough but short mid-level adventure.
Winds of the Ice Forest - Low level adventure for Labyrinth Lord. Great art and includes a barbarian class for LL
Pay What You Want Sourcebooks
Dragon Union - Very strange.
Guidebook to the City of Dolmvay - Big city sourcebook to the city in the back of the Labyrinth Lord sourcebook.
Teriatic Tome by Rafael Chandler. A little tentacle monster heavy, but good.
Morgajel - Makes all sorts of free mapping resources for GIMP
Small Publisher Guide from Kevin Crawford, who has done more to promote do it yourself RPG creation than any other person I can think of
Game-Icons.Net - Free game icons
S&W Monster SRD - Contains stats for many creatures from the Tome of Horrors, which is nice since it's insanely expensive
Open Clipart - Free Clipart. Quality is not great, but a few nice pieces, as well as lots of swords and such.
Teazla's Free DND art - A collection of public domain images as well as some Larry Elmore clip art originally from a cd-rom he put out about 15 years ago
Public Domain Pulps
In the good old days, copyright was not forever. This has allowed quite a large number of old pulp magazines to go into the public domain. Besides the stories still being pretty decent, there's a lot of great artwork that can be extracted. Most of these are from ComicBooksPlus, but PulpMagsProject has many of the same issues in PDF format
Fantastic Adventures - Hosted at Comics Books Plus, this is is a couple dozen issues of a magazine focused on fantasy of that era, which is generally stuff in the past or lost races stuff
Planet Stories - This magazine is more focused on planetary romance/sword & planet
The Thriller - This is actually a UK cousin to the pulps, the "British Story Paper". Actually is more about noir and crime fiction, great if you need art for a CoC game
Weird Tales - I often wonder what would have happened if H.P. Lovecraft actually accepted the job as editor of this magazine? Probably would have been a lot weirder. Still a lot of good stuff in it and great artwork.
There's also a lot more pulp stuff at the Internet Archive, but they aren't so fussy about copyright there, so I would be wary of using art from mags taken from there.