Well to start, emulation. Styles/customizing. vlc has been ported (and it's amazing) pocket touch is amazing for when you can't look at the screen. a great web search program. a last.fm scrobbler that automatically scrobbles your songs as you listen to them. There's an app called mxtube that allows you to download youtube videos to watch them whenever you want. swaptunes lets you sync multiple itunes libraries with your iPod touch. comicviewer is a great comic book reader. dock is a really cool way to switch applications while you're in other ones. appflow, I entirely love this app so much, I can hide all of my applications but a few, and use appflow to use cover flow to go through my apps and launch them. bossprefs just does a lot of things that are great for your iPod, including hiding the applications like I said earlier. five icon dock, does exactly what it's name says, it lets you put five icons on your dock. I love it!
That's just what has been useful for me, I understand that for a lot of people they don't think it would be worth it. but I'm the type of person who wants to get every little bit of functionality out of my devices I can.