Appshopper is great. You can find a lot of bargains, like apps that have temporarily had a price reduction. When a new or revised app comes out, the developers frequently lower the price for a few days so people will try it and post reviews. So, you can often get apps for free that will have a cost later.
Two apps I really like that have not been mentioned yet are Gaia and Ambiance. Gaia is a tile matching game that takes advantage of the iPod touch /iPhone ability to know what position it is in to slide tiles in place. It's a good game but, also relaxing.
Ambiance is a relaxation app with relaxing sounds. Some of the sound relaxation apps have a limited number of sounds built in that play on a loop. Ambiance allows you to choose the real recorde sounds (almost all free sounds) that you want to download and if they loop, it's not noticeable. I like the walk on a rainy French forest for working to and someone I know likes the coffee shop.