Mobile mouse - use as a trackpad for your computer, handy when watching tv online.
Instagram - free photo sharing with effects
Pandora - free streaming music
Red laser - see a barcode, read it. Very handy for all those qr codes advertisers are using.
Amazon - want to curb an impulse purchase at the store? Scan the barcode with this app and find out how much less you can get it on amazon for.
Good reader - if you have a lot of PDFs you like to reference, this is hands down the best tool for reading them. Integrates with Dropbox, I just store all my PDF books and documents there, and pull them into good reader when I need them. You can also annotate PDFs, read word documents, and a bunch of other file types.
Kindle, audible.com - books and audio books. Amazon now has a list of their free kindle books, it changes daily and they are generally very good. Theres no reason to ever be bookless now.
The monster at the end of this book - two versions of this sesame street classic. Keep It around for those times you have to entertain a wee human.
Netflix, Ted - two great sources for visual entertainment and enlightenment.
Relax melodies - the best white noise relaxation generator I've used. You mix and match looping noises, like rain, ocean, and wind chimes. Control the volume of each noise individually, and all of them together. Save your favorite configurations. The free version is good enough, though if you really need twice as many noises the pay version is more encompassing, and worth it for the additional alarm and sleep settings if you use it a lot.
If you walk or run, itreadmill is a good pedometer. The iPod touch doesn't have a gps, so many apps for running, cycling, and so forth won't be much use unless you like logging things by hand. Itreadmill doesn't give you a route map, but does give a pretty good estimate of pace, distance, etc for walking or running, and graphs it so you can manage a regimen.
NPR and NASA have apps if you're into either of them.
If you've ever had an itch to broadcast live video online, ustream has an app for that.
If you use timers to force yourself to work on task for short bursts of time, the are numerous pomodoro timers.
If you like to track packages frequently, posted is a good app for seeing the status of multiple packages from many services all in one spot.
Register your device with iCloud and turn on find my iPod, then sign on to every wifi hotspot you can in all the places you normally visit. If you leave it behind in one of those spots, you can use iCloud to figure out where it is, send it messages, etc.
Multiponk is a multiplayer pong game, great for passing a few minutes waiting for something with friends.
Ragdoll blast, Ragdoll 2 are both awesome fun.
Boxed in and boxed in 2 if you like puzzle games.
Words with friends - great scrabble implementation, and works with their facebook app so you can play with family and friends that don't have mobile devices. Great way to keep a connection with someone who lives far away that you have no reason to call, but you still want to maintain some level of "hey, I'm still alive" type communication.
Get in the habit of visiting
http://freeappalert.com and pick up apps that look interesting for free.