Here are two games to play in your web browser and destroy your productivity:
Desktop Tower Defense. Action! Guns! Squirt Towers! Recommended by Jeremy Zawodny.
MathsNet Interactive Geometry. Three dimensional puzzles! Tip: Puzzle #1 is the hardest one by a mile. Recommended as retaliation for the earlier link by fpereira.
Go play them! Just don’t blame me when you get fired.