Dead Meat by Sean Wipfli - I hate to start this list with a game that appears to no longer be available for download, but Dead Meat deserves some recognition. It's the sort of RPG that a confident GM could run as a party game for non-roleplayers. The golden idea in this one: all players get three PCs, because the game assumes everyone will die at least 2 times per session!
Puppetland by John Tynes - Shit, is this one gonesville, too? I'm getting old and out of touch with my online freebies! Fortunately you can still find copies of the commercial release published by Hogshead. Anyway, Puppetland was the first game that really creeped my shit out in a good way. I'm not sure if I'll ever run it, because A) it looks like hard work to actually play and B) it gives me the jibblies.
Powergame by Mikko Kauppinen - Finally, one you all can actually download! Powergame is a supers game that takes the basic lighthanded approach of the old Marvel FASERIP game and ditches the big percentile chart in favor of small d6 dicepools. Nothing mechanically revolutionary here, just a solid design that gone through a lot of playtesting and rewriting. Years of chasing the mechanically perfect superhero simulator have left me jaded, but Powergame is one of the few supers games I still give a crap about these days.
Bad Attitudes by Uncle Bear - Shitburgers, this one seems to have gone AWOL as well! A nifty little uber-simple game devoted to action movie shenanigans, with a silly random adventure generator. Not enough game for a campaign, but makes for nice explodey one-shots.
Wuthering Heights Roleplay by Phillip Tromeur - I ran this at a con once and it was an absolute hoot. Overwrought period meoldrama is pretty much the polar opposite of the kind of games I normally run, but Mssr. Tromeur's design makes it a fun change of pace.
Perfect20 by Levi Kornelsen - One word: tight. More than one word: Does everything I want from a generic d20 light system, the 'build your own class' idea behind the advancement traits system is exactly what I want from a d20 game, and even the frickin' layout is perfect. A 'go-to' game every bit as awesome as slick commercial releases like my beloved Savage Worlds. If Levi sold this beauty as a softcover on Lulu I'd be first in line to buy one. Heck, I'd consider buying extra copies to hand out at my game table.
Mazes & Minotaurs by Olivier Legrand - This game is basically a re-imagining of original Dungeons & Dragons with two key differences: the setting is mythical Bronze Age Greece and the mechanics actually make sense.
FUDGE by Steffan O'Sullivan - The original toolbox RPG is still one of the greats, in my opinion. Am I the only one who thinks it's funny when gamers complain that you can't play this system without owning special dice?
The S. John Ross Triple Threat: Risus, Encounter Critical, and Pokethulhu by S. John Ross aren't just some of my favorite free rpgs, they're some of the best damn roleplaying games I've encountered. Full stop. Risus is the rightful heir to the kind of comedy gaming fun that I used to get out of Toon and Ghostbusters. Encounter Critical distills down everything that's awesome about Arduin, Rifts, World of Synnibarr and other crazy brain kitchen sink rpgs, leaving you with Over-The-Top Gaming Without Tears. Pokethulhu mashes together two jokes that have grown tiresome and unfunny to me, yet the resulting combination still cracks me the hell up.
Dungeons & Dragons - Sure, you can throw an infinite amount of money at the D&D, but these days you don't have to. The various online SRD sources make playing without corebooks a destinct possibility, though I find having the hardbounds at the table to be convenient. And then there's OSRIC, which makes 1st edition AD&D available as an Open Game License product. There's nothing I could say or do that would change the mind of anyone with firsthand experience of AD&D, but if you've never seen the inside of the old tomes following that last link may allow you to have a more informed opinion of the Bad Ol' Days. Basic Fantasy is also a good option, as it basically gives the OSRIC treatment to Basic/Expert D&D, with a few changes. 'Elf' isn't a class under Basic Fantasy, but otherwise it's pretty much on-target.