Antbuster, SON.
This game is driving me nuts, it keeps track of your scores vs other players, and even groups players by country. (USA! USA!)
Its a really simple game by appearances, not great graphics or anything, but once you figure out that by just randomly placing and upgrading guns you level out at like level 50, it really gets interesting. There are tons of gun choices in three classes (power, quick, spread) and there are specialty guns hidden (laser towers*, missle towers, shock towers, pulse towers, sasha vujacic guns (machine), fire and ice towers) and just tons of planning and patience involved. My best score is level 84 right now but I will get to level 90...Im just not sure how yet....
www.rstein.org Just click on the big green graphic in the middle that says AntBuster. Not recommended during business hours for risk to job security.
*Laser towers are over-rated.