Everyone will have different opinions regarding your question, but here's a few of my "must haves".
Wayfarer (browser)
Iris (email client)
WookieChat (irc client)
MPlayer (media player)
RNO Radio (streaming radio)
Grafx2 (image creation / manipulation)
EvenMore (text/document/html viewer)
MCAmiga (file manager)
SDLPoP (best game ever)

Cheers