Spiral Knights on Steam is really addicting, until you realize you have to constantly pay to play with real-life currency if you want to get the best weapons/equipment and play more than one dungeon a day. If it wasn't for that, I can see it being one of my favorite computer games.
Altitude can also be pretty addicting. It's one of those games you mean to just play for a few minutes, and then end up playing for much longer.
EDIT: Oh, and how could I forget Sim City? There's a sense of satisfaction to watching your town grow bigger and bigger as you play more, then destroying it and starting again.