Yes, this should be in the App's section, but I'll leave it here for a while to get it rollin'.
Essential App's... it's late so I'll add just one.
PocketTunes: the definitive MP3 player - allows you to skin your player and the deluxe version allows for streaming audio broadcasts.
Okay..two.. Snappermail: Rock solid email application
Galaga (who doesn't love this?)
I'm a big Galaga fan, but haven't tried it yet on the Treo.
Looks pretty cool though... nothing quite like vintage gaming.