Matt Finch has a great idea.
I lurked for two or three years on the blogs before I started this, so I've amassed quite a pdf collection on my hard drive of free things. Here's a partial list:
Christian over at Destination Unknown pumps out a crazy amount of resources. My favorite, and the one I use the most, though, is his Freecity of Haldane setting. The really amazing part? He mails this stuff to you if you ask him to! In my opinion, Christian doesn't get enough thanks for what he does, probably because he does most of his contributing off-line through snail-mail.
Michael of ChicagoWiz's RPG Blog gave us his Swords and Wizardry Quickstart. This was one of the first modules that I ever ran and it contains a lot of good help for new referees.
Speaking of the first modules I ran, I'm pretty sure that my first module was this one page dungeon by the people who publish Dungeonslayers. They have a lot of other free resources too, like other one page dungeons, some rules supplements (firearms!) and, of course, their own RPG. While I don't use Dungeonslayers, I like that it uses the Metric system. That way I don't have to convert its measurements when I run modules written for it.
Telecanter has also put out some really cool stuff. My favorites are probably his Choose-Your-Own-Rogue class, which I've been using in my Skype campaign and his Alabaster Tower module- the first module that I've run twice. He also has a module that will both freak your players out and serve as a really cool introduction to an underground campaign in an evocatively described "Undersky." This module alone made me want to create an Undersky setting, just so I could kick a campaign off with this module.
Setting-wise, I'm also heavily indebted to Will the Coffee Swillin' Analog Gamer for his setting of the Barony of Northmarch. Along with Haldane, this is what has allowed me to run a sandbox campaign without very much prep at all.
Of course, thanks need to go to those who contributed One Page Dungeons to the contests these last three years. When I need a dungeon, these are the places I usually go to get them. In particular, Christopher of A Rust Monster Ate My Sword and John of The Nine and Thirty Kingdoms created really cool dungeons that provided multiple sessions of play.
Finally, the blogger who's post inspired the post that inspired this blog, Courtney at Hack and Slash, who's document and format for traps provided a way for me to make traps evil and fair at the same time. I've also started using his Out of Dungeon Living Expenses document.
And that's, I think, a good list of the less-obvious people who need thanking for things I use regularly in my games.
This whole issue, though, has got me thinking. I really do owe a lot to the online OSR community. It, more than anybody face-to-face, taught me what I know about the Old Ways, and it's provided me with the free stuff that makes up the vast majority of what I use in-game. I'm really thankful, but maybe I haven't been expressing that enough.
So here's a resolution: when I download something, I should comment as well to say "thank you." When I use it in a game, I should return to say "thank you, I actually used this and it made my game better in this way." That really doesn't sound so hard, and yet I've been lazily, un-gratefully not doing that. Time to change that, starting with everyone on this list.