firefox - simply because safari won't let me access my college email account. i've never known why, but just live with it.
Exposé - i know it's not really software, but i could not live without it. with no taskbar, like xp, it's hard to know what windows are open, but exposé rules out the need for a taskbar, and is much cooler.
i'll get back to this thread when i'm on my mac and realise there's something i always use!