MacMerc's column on More Menu Extras points to tvMenu which shows your local tele in a menu, and can alert you when a selected show is going to start. Nice! Shame it's US only ("A worldwide version is currently being developed" -- XMLTV might be worth a look for that. I use it for my own tiny tele listing app).
Acquisition [thanks Tom C] is a Gnutella client with a lovely interface. It works really well with iTunes: when a track's finished downloading, it adds it to your music library, and pops it in the Acquisition playlist so you can find it. Oh, and it's dead easy to use, with subtle hints to show you what's going on, and where.
It's difficult to decide on the coolest thing about Ask the DJ [via mrbarrett]. Not only does it automatically figure out BPMs to your tracks, not only does it then magically mix them together, all the while talking nicely with iTunes... But it looks like a CD. Plus the cartoon lady dances when the music's on.
This is why iTunes comes with the OS, isn't it? So that apps it would be nice to work with a music app/library actually do work with a music app/library, because the API is known.