Gotta add VLC media player - for when nothing else will play that dodgily encoded episode of Lost you downloaded...!

Also, MacJanitor is great for maintenance stuff, especially if you have a laptop. And check out Comic Life - a fantastic little app that let's you make comic strips from your own photos, complete with speech bubbles, captions, etc. Great fun!

Oh and Stuffit Expander is pretty much a must have for all those who've recently switched - you'll come across .sit files everywhere in Macland.