To keep track of the ever changing list of applications for the iPhone and the iPod Touch AppShopper is a great resource.
As you look across the top of the page you may choose to search on All, Paid, or Free apps. There is also a button for Updates, New Apps and Price Changes.
One of the more convenient ways of keeping track of new applications you may be interested in is to use RSS feeds which will alert you of new changes.
The RSS Feeds come in a variety of choices as well. You may choose to be alerted to all changes in the app store, only new apps, only paid and updates as well as price changes have their own feed.