Apple's remote app is pretty nice if you use iTunes a lot.

Brian Eno's got an app called 'Oblique' which basically is a tool to get over writers block, but you kinda have to see it to get what it's on about.

Bookworm (ok, so it's a game, but all the same!)

Twitterific (if you're into twitter)

Evernote is a pretty decent note taking app that links in to their online service. Probably only useful if you already use their stuff.

I don't use my iphone as a ipod but if you use yours for music then last.fm is probably worth checking out.

dryad_wombat's got a bunch of stuff on her iPod touch which might be useful, I'll see if she feels like posting about anything she's using.