Well because I use my iBook for everything, I'd have to say my Macally 2 button+scroll wheel Micro Mouse. If you're going somewhere the iBook goes under your arm, the mouse in your pocket.
Favourite thing that connects to my Mac...how about ME!
ok ok...my iPod was at the top of my list until I realised how crappy the 5 gig model is compared with the new ones. I'd have to say my digital camera, I use it all the time.