The free sports tracker on the downloads page is pretty nifty.
Opera Mini 4 beta is a solid browser, and has its strengths over the Symbian browser, but isn't necessarily better.
MGMaps - Mapping software that uses Google Maps, MSN Maps, Yahoo Maps (and satellite). In theory free navigation, but doesn't work for me.
Google Mail - good if you have a Gmail account, and arguably better than using the native email client.
As above, Fring is good (although it has its quirks).