The scrabble clone Scrabulous is great way to waste hours. You can try to work your rating up in competition with other players in timed games. It's pretty addictive.
Desktop defense is very popular and highly addictive as well. It takes awhile to get really good at it.
Onslaught is also interesting. But it pushes a bit beyond what flash games are capable of. It seems to completely peg my CPU, and still slows down noticeably when there are 400 or 500 little monsters working through my kill-zones.
Read the instructions before you play too long, trust me.