shazam, google sky maps, flixster, key ring, shop savvy, Advanced Task Killer, handcent sms if you text a lot.
Tune wiki is really cool, but I'm not that into the music/social networking or having lyrics and cool interweb tie-in stuff when I'm listening to music. I'd rather preserve the batteries.
Layar has the potential to be really awesome, but I haven't found a use for it yet.
I don't do the social networking thing, but meebo is supposed to be really good.