Here is an interesting one I ran across today: Wikipedia on your handheld or cellphone:
http://www.en.wapedia.org/According to the site:
Having started in August 2004, Wapedia brings the contents of Wikipedia to mobile devices like mobile phones and PDAs. It offers the recent version of every article, which is done by using a combination of a proxy-like behavior and a local article database. This combination provides fast speed and up-to-date articles on the one hand, and a low load and low traffic for the Wikipedia servers on the other hand.
It comes in 18 languages, if I counted correctly, but you only access the information. The real breakthrough would be to actually be able to edit or create entries in Wikipedia right off of your mobile device....