Because this thread is titled "Time Wasters" and not "Flash Time Wasters," I'm equal opportunity when it comes to productivity killers.

Biolab Disaster - I'm also a sucker for 2D platformers, as long as the controls and gameplay are good. This one is good enough, even if it's an extremely basic and very short game. It's also done entirely in HTML5, and those games are starting to have a sort of vibe to them that's hard for me to explain. Maybe it's the resurgence of sprites or something.

Anyway, like I said, it's really short, but it might be fun to try speed runs.