In case WoW isn't addictive enough for you, I present Tower Defense:
http://novelconcepts.co.uk/FlashElementTD/
There's another interesting variation at:
http://www.handdrawngames.com/DesktopTD/
If you play the latter, I've created a "score group" called Eventide. Just specify "Eventide" after you obtain a score and you can compare it to the rest of us.