I'm not so sure about free...if you're looking into that category you shouldn't expect to find a lot...
if you're into the single player aspect and not so much the multi-player and you want it free...you'd probably head on over to the "dark-side" and get your mittens on some "cracked" copies of popular titles from the web...*Warning* Although you can acquire full working copies of great games of the web, remember...by downloading you are partaking in the pirating of software, running risk of spyware/virus/malware infestations, risking system stability, killing kittens, and limiting yourself to only the single player campaigns(most pirated games won't let you play online) not to mention how frustrating it may be trying to decipher crappy instructions, extract corrupted archives, running into errors, ect.
however, assuming you're not cheap to the point where you can't afford a ten dollar game..
You can find tons of great PC games at your local retailers for relatively cheap prices, I personally know you can get yourself some decent FPS's at your neighborhood Walmart such as Half-Life, Call of duty, Medal of honor: Allied Assault and Pacific Assault , Battlefield:1942, Battlefield 2, Starwars Battlefront 1&2, Starwars Republic Commando all for about less than Ten bucks a piece. They're all great games, cheaply priced, and not too demanding of your machine
I would also like to recommend one of my personal favorites: "Fable"
It is and awesome Third person action RPG, that looks and plays great, but it is quite short( you can clear the main missions and complete the game in about 4-5 hours)
You should check some of those out, they are well worth the ten bucks:]