Defense Grid: The Awakening ...is beyond addictive!
Sometimes you only have a few minutes to get into a game and have some fun. In this regards an MMO like EVE Online might not do the job (though that may soon change with the mini-games in WiS like SecWars). That where a good tower game like Defense Grid can be a god given! I came across this treasure of a game reading my latest CPU magazine. The guys at Hidden Path have don't a great job and the rush you get as you near the completion of a level is priceless!
At $20, the game is a bargain (I got it through Steam at $5 during a limited time offer I imagine).
If you're looking for a simple strategy game that quickly pulls you in, then Defense Grid: The Awakening is it!