Onslaught
Addicted to Desktop Tower Defense yet?!? Spend hours and hours of your employer’s time building a maze to trap those creeps? Me, neither. 
You're not going to like me for this. Desktop Tower Defense isn't going to like me. I found a new addiction. This new addiction is a Tower Defense game called Onslaught, and it's got a twist. The creeps in Onslaught follow a specified path, no building mazes in this one. The goal of Onslaught is to maximize your firepower. You can cluster different groups of weapons together to form a super weapon. This game is sure to let the hours slip away unknowingly. I recommend giving the tutorial a run through so you can learn how to build up the weapon combos.
Have fun!
Link to Onslaught
[via Just Whatever]