Onslaught

If you have a day to waste, consider wasting it playing Onslaught. It's similar to Desktop Tower Defense--wave after wave of enemies comes flying at you, and you need to build up a weapons network that will stop them from reaching your base.

My best performance so far has gotten me through level 475 or so on the Easy skill level. Apparently, that's not very good, although it's good enough to "beat" Easy.

It's fun and mildly addictive. I don't want to know what happens if a person plays it at work. Probably nothing good.