MapQuest provides two different API's: one for open use and one for licensed use. Both require a key, so I don't know if there's any difference in the function calls besides the key? Directions, Search, Geocode, etc are available and provide a free alternative to Bing.
The terms of use actually state: "...any commercial application not publicly available without charge (other than mobile applications for which users pay a fee to download/install the application)" Our software is available for public download without charge so this is not a problem.
Using MapQuest for any commercial application is prohibited. Read more in their Terms of use page: http://developer.mapquest.com/web/info/terms-of-use