Left 4 Dead- Really fun. They are coming out with new DLC on April 21st.
Fallout 3- You will easily kill over 130+ hours if you play all the side quests. This doesn't even include the new Pitt and Anchorage add-ons.
Counter Strike Source/1.6- Still a classic in my book. You can play online with the endless amount of maps they have.
Crysis Wars- The Crysis series hasn't exactly been fun. Great game online. It is taxing on your system.
Test Drive Unlimited- Takes a while to get through. Plus, you can play with a lot of people online.
World Of Warcraft- I'm personally not a fan. It's too hardcore for me. I hear it kills a lot of time. I found it to be very boring.
Starcraft- I'm not a fan of RTS. It can be a fun game.