Basically, a lot of the games that people have suggested are great and you should also check out FIFA Online which has open beta now and Need for Speed World.

Shattered Horizon $9.99 on steam until Sunday (June 6th), and also free to play this weekend June 3rd - 6th

Also, Battle of the Immortals is my favourite free MMORPG.  
 
If you like Diablo, check it out.

2029 online

" Shattered Horizon $9.99 on steam until Sunday (June 6th), and also free to play this weekend June 3rd - 6th "

Dude...Killing floor is a great shooter for the PC and it's cheap.  Nothing like shooting Zombies with a Grenade Launcher!!!

How About Stepmania It's like Dance Dance Revolution,and it's free. and since you have source games you should get Source SDK and play some source mods like Obisidian Conflict,some mods are total convertion.
 
There are a lot MMO out there,so many that i can't possibly list them all myself. you could try check the site that dedicated to MMO like mmosite

League of Legends is free and awesome. 
 
And Minecraft is really fun. The paid version is where it's at. 

I recommend the game Alien Swarm, absolutely free, the creators of Half-Life.
http://store.steampowered.com/app/630/

Ben There, Dan That... 
 
http://www.zombie-cow.com/?page_id=17  
 
Fantastic donation-based throwback to Lucas Arts adventure games.  

Red Alert 3 is $15 till Sunday on the EA store, anyone interested in RTSs should definitely get that one.

Alien Swarm is free and is really cool.

Duders, check this out.  It's a wiki page for all the games once paid for, now freeware. 
Among the notable releases I've seen are - 

• Daggerfall/Arena of The Elder Scrolls series fame  
• Command and Conquer/Red Alert/Tiberiun Sun 
• GTA/2 
• Ground Control 
• Mech Warrior 4  
• Psi Ops  
• Sim City  

I can recommend the site http://www.gog.com. Lots of cheap, good old games :)

try some indi, it's free!

• Iji
• elona
• Cave Story
• Dwarf Fortress
• AI: War (not free)
  

Wow, surprising that none of the games in the Chzo Series of adventure games made by Yahtzee have been mentioned yet.
 
Four games, 5 Days a Stranger, Trilby's Notes, 7 Days a Skeptic, and 6 Days a Sacrifice
 
All awesome, and providing some good scares.

-Shooters: CombatArms is pretty cool for a free game, Americas Army is nice if you have ppl to play with, BF heroes turned really crappy recently so I cant recommend it anymore. 
-MMO: Stay away from LOTRO cause for what I´ve seen in the F2P beta it´s going to be even more expensive than paying a subscription, Runes of Magic is ok, but it´s way too ugly for me, there are a lot of free mmo´s but most of them are crap, there is a good site that deals in this genre exclusively... MMOhut.com 
-RTS: Battleforge is cool but gets boring after a while if you dont put cash into it, Im looking forward to Age of Empires Online... 
And many others that Im not remembering now, if I think of something else I´ll add it.

All of the aforementioned games are great, but honestly, just get Steam and follow what deals they have going. They always do this thing starting Tuesday nights, it's called Mid-Week Madness. They always have really good deals on games, some of the games are kinda meh, but you can pick up some awesome one's for dirt cheap. This week the game is Plain Sight. I haven't played it, but I've heard decent things about it and it's on sale for 2bucks. At that price who gives a fuck if it sucks! Anyways that's just my two cents. 

Alien Swarm on steam is great, and free. Quake live is the best retro multiplayer shooter experience you can get these days. Canabalt. Super Mario Crossover is great. Moonbase Alpha is pretty cool, and Runes of Magic is the best playing/looking free mmo ive seen to date, though i've heard good things about runescape.

Knights of the Old Republic is $2.50 on Steam today. GO BUY IT!

" Knights of the Old Republic is $2.50 on Steam today. GO BUY IT! "

Week-long THQ sale on Steam this week. 
 
Today's games are all three Red Faction games, €7.50.

Stalker: Shadow of Chernobyl and Call of Prypiyat are now $10 as a bundle on Steam. 
 
Amnesia: The Dark Descent is $10 on their web store, $13.33 or something similar on Steam.

OMG, OMG, zombie thread!!!! 
 
but yeah baseball fever and mlb 2k10 is like 2 bucks

Play Vindictus.

Urban terror 
Most epic game I have played it for a year and a half and I still keep coming back to it :D 
I play almost everyday

" Urban terror Most epic game I have played it for a year and a half and I still keep coming back to it :D I play almost everyday "

" @NekuSakuraba: the wonderful word of the web "

" @Ryax said:

" @NekuSakuraba: the wonderful word of the web "

In the time you made that for me, you could have done a funny dance.  Fail? or WIN? "

Dwarf Fortress
Space Station 13
ADOM
Nethack
 
When you're done with those in a decade, let me know and I'll make some more suggestions.

Today, Age of Empires 3 Complete Collection is $0.10 
 
http://www.gamesforwindows.com/en-US/dailydeal/day7/

" Today, Age of Empires 3 Complete Collection is $0.10 
 
http://www.gamesforwindows.com/en-US/dailydeal/day7/ "

" @Geno: Is it a misprint? What is going on here? The bandwidth must be costing them more than ten cents... "

" @Geno: Is it a misprint? What is going on here? The bandwidth must be costing them more than ten cents... "

" @MB said:

" @Geno: Is it a misprint? What is going on here? The bandwidth must be costing them more than ten cents... "

Well this is inevitably going to be reported to every corner of the interweb...the publicity for GFWL would be worthwhile I believe. It's also a show of good will from MS to PC gamers. I've personally bought Gears of War and Viva Pinata to show my appreciation (not to mention they're worthwhile games anyway).  "

" @Geno said:

" @MB said:

" @Geno: Is it a misprint? What is going on here? The bandwidth must be costing them more than ten cents... "

Well this is inevitably going to be reported to every corner of the interweb...the publicity for GFWL would be worthwhile I believe. It's also a show of good will from MS to PC gamers. I've personally bought Gears of War and Viva Pinata to show my appreciation (not to mention they're worthwhile games anyway).  "

Yup, just saw majornelson post this on his twitter.. I'm thinking of buying like 10. Just because I can. "

I'm putting together a big list of Cyber Monday deals on PC games on my blog. 
 
Part 1 with GameStop (and O'Reilly books): 
http://blog.charlaxian.com/2010/11/cyber-monday-deals.html  
 
Part 2 with Amazon and BestBuy: 
  http://blog.charlaxian.com/2010/11/cyber-monday-deals-part-2.html 
 FYI, I used Amazon affiliate links, and interestingly the price is lower on a few of those than the price listed on their main page. 
 
Also, I've been covering Steam every day: 
  http://blog.charlaxian.com/2010/11/steams-give-and-get-sale-day-five.html  
These prices are good through 10 am PST.  If Steam has more sales today, I'll do a new post on that.
 
And here's something on D2D's promotions: 
http://blog.charlaxian.com/2010/11/d2ds-holiday-giveaway-gimmick.html     
 
Also, I'm watching D2D, GOG, etc., to see if they have any special sales.

Here's another post, wherein I shovel through all the shovelware on sale on GamersGate in order to pick out the deals someone might be interested in: 
 
http://blog.charlaxian.com/2010/11/cyber-monday-deals-part-3-shoveling.html  
 
Some highlights include:  
Alpha Protocol - $7.49
Batman: Arkham Asylum GOTY - $10.19 
Deus Ex Collection - $4.99  
Eufloria - $7.99
Evil Genius - $3.98 
Torchlight - $4.99 
Tropico 3 - $4.99 
World of Goo - $2.49 
X Superbox - $15.99 
Zombie Driver - $4.97 
 
There are some other games on there for under $5 as well.  Some of their titles are so bad that I'm tempted to try them for a laugh. 
 
Update: 
 Today's Steam sale was just posted.  I've reviewed five of the games (Altitude, DogFighter, Lead and Gold, Plain Sight, and Trine), so if you're unsure about them, take a look. 
 
http://blog.charlaxian.com/2010/11/steams-give-and-get-sale-day-six.html

Probably already been mentioned before, but League of Legends is free. And epic.

gog have started their christmas sale 
http://www.gog.com/en/page/xmas10/

" I recommend Warsow.  It's a cel-shaded UT style shooter, and it's really high quality. "

" gog have started their christmas sale 
http://www.gog.com/en/page/xmas10/ "

http://www.gamersgate.com/   
 
cryostasis is like 3 bucks today.  the other day bioshock was 5 bucks, dead space 7, and stalker clear sky 4