RuneScape - the massive online adventure game by Jagex Ltd is by far the coolest one i've encountered. this is only for fans of RPG's and/or nerdy D&D type games. Have an uncontrollable urge to fight giant spiders, quest for flour, or save sheep from a deadly monster? This be the game for you! It's pretty involved, so much so you have to sign up for a free account (helps because it has an auto-save option that saves a lot of time and frustration) and it's best if you have a very fast connection. There is a pay for play option to the game, but odds are you won't be able to finish all the free quests anyway.
For the sports fan, there's the 2 Minute QB Challenge. Extremely addicting if you've ever wanted to command a team to victory in the 4th quarter.
The cutest game on the web right now has to be HAMPSTERBALL! This is just good christian fun here. What more can i say!
And for you evil genuis types out there, there's the Flipword. This game is REALLY fun if you have spell check, very frustrating if you think words like 'qwerty' and 'cazf' will count.
So what's your favorite of the four? Leave a comment and vote!