WiFinder is essential; a much better interface for finding Wifi signals floating around.
Tweetdeck for Twitter is great if you use that. Syncs with desktop client.
Comics by Comixology is cool, it's like an iPod comic book store with lots of free stuff.
Baseball Superstars is probably the game I play the most on my iPod; I have '09, but '10 is out and I've heard that's better. Actually pretty much any game by Gamevil is worth grabbing.
AuroraFeint (and related games like Tower Puzzles) is another fun game.
There are some cool Final Fantasy tower defense games like Crystal Defenders and Vanguard Storm (the second one is like Plants vs. Zombies).
You can also grab Tyrian, it's been ported.
Amazon's Kindle app is pretty nice; they usually have a couple of free books to download.