Everyone loves free stuff...
Games sure are expensive aren't they? You don't need to answer that, I can't hear you anyway. Despite this I'm giving you a list of the best 8 free games you can download for your PC. Here it is:
1. Ace of SpadesCredit: http://ace-spades.com/wp-content/uploads/player.jpg
In a world... Made of cubes... A war like no other rages. In this online first person shooter the fight really is different. Everything is made out of cubes, but what makes this game unique is the ability to build the levels as you play. By placing and destroying blocks you can dig trenches, tunnels, build bases, or pretty much anything you want. The whole level can be changed, and over time it will be. The goal is usually to capture intel from the opposing team, though some servers run customized game types like team death-match. It's still in development too, so expect new releases to be better and better.
2. Battlefield Play4FreeCredit: http://www.gamezone.com/images/sized/images/screenshots/battlefield_play4free-400x225.jpg
If you've played any Battlefield game before you'll feel right at home here. Like Ace of Spades, this is a team based first person shooter, though this one has a higher budget. You can fly helicopters, drive tanks or trucks, run on foot, or even fly jets. The vehicles and large maps/player count are what makes this game worth mentioning as it really helps it to feel like a battle. There's an option to buy in-game weapons with real currency, though it's not at all necessary.
3. Team Fortress 2Credit: http://www.tfportal.de/gfx/content/tf2/classes/class_sel_sm_heavy_red.png
Maybe the most polished free to play game out there, Team Fortress 2 is amazingly fun. Originally costing money, it's now a free stylistic online first person shooter that is worth checking out by any fan of the genre. Teamwork is the name of the game (in case that wasn't obvious), and with 9 different characters to choose from it's important to find your niche(s). Every character plays differently and every one is fun. This also has the option to purchase in-game object with real currency, though it's more of a gamble and also not necessary. Also plays on OS X.
4. Cave StoryCredit: http://fc08.deviantart.net/fs43/f/2009/105/9/d/Cave_Story_dude_icon_by_Doctor_Cool.png
You may recognize Cave Story as a game on Nintendo's WiiWare service (which is accurate recollection, good job), but before that it was a free game on the PC. It still is in fact, and it's probably the best platforming adventure game since Metroid. It's professional 16-bit-style graphics look as good now as they did when it came out, and it's gameplay is classic. If you're a fan of these types of games: Play it.
5. ToriBashCredit: http://tk-nation.com/uploads/1_toribash2.jpg
Cave story isn't the only free PC game to have found success on Nintendo's service. Toribash is a free online fighting game for PC, Mac, and Linux, and it's more original than any other fighting game in the last 5 years. Every move is based on how you control each muscle individually per instant. It has keyboard shortcuts to facilitate this process, but only a few are needed to compete. The best thing about this system is there are no set moves. Everything is physics base, from the damage your punches do, to the movement your character needs to take in order to double ear slap your opponent into submission. Comes with tutorials, and a practice mode to get you started. Also you can dismember limbs (worth mentioning).
If 8 bit games are more your style, you may like La-Mulana. Made by the folks you brought you cave story this game features a low-res whip wielding Indiana Jones type character on a mission for treasure. It's a challenging game that requires precise timing in your jumps and attacks.
7. Noctis IVCredit: http://www.gameclassification.com/files/games/Noctis-IV.jpg
Probably the most obscure title on this this list, Noctis isn't a game in the sense that you can win, or lose. There are no points, or money, or unlockables, and none of this makes it any less amazing. The premise is: You are a feline-esque space explorer. You are alone in the universe traversing your box shaped space craft from planet to planet looking for fuel and anything else that you may find interesting or worth exploring. The galaxy you explore has millions of planets and stars to see all rendered in glorious 320 x 200. Originally designed for older systems, you can still get it running just fine on windows XP. Despite the low resolution and low-poly graphics Noctis has a loyal fanbase, and the members of which have created their own programs to catalog the discoveries, or updated the game based on the source code.
8. Dink SmallwoodCredit: http://www.dinknetwork.com/images/characters/duck.gif
Feeding pigs, punching dragons, insulting random people/book cases – It's all here in: Dink Smallwood! This game is great (and to put it as bluntly as that seems to be the best I can come up with). It was the first game I ever bought, and will be the game I'm buried with. Maybe the best thing about this adventure RPG is it's humor, which is always well timed and well worded. After that would the the hundreds of mods users have made, extending the life of this game for hours. Lots of hours.
Any one of these games could keep you busy for days, and I recommend giving them the chance. You may find something better to do during office breaks than solitaire, or facebook.