There are a bazillion apps available for iPads (slight exaggeration) and quite a few that are useful to educators. The Texas Computer Education Association has created a Goggle spreadsheet to organize educational apps and they have been kind enough to share this document with the rest of us.
Apps are sorted by subject to make it easy to find something in a specific area. Prices are included in the spreadsheet, free apps are in white.
I learned about this nifty resource at Apps In Education
iPads & Education 