Have a look at Mapquest's geocoding API.
Do check out their Terms of Use, though. Here are a few of the requirements:
- OpenStreetMap must be given credit for the data.
- Please place “Geocoding Courtesy of MapQuest ” on your page, or in the copyright area of the map and please link the word “MapQuest” to http://www.mapquest.com.
- If your application will get heavy usage, please let us know by sending us an email at open@mapquest.com. Please include the estimate of your expected usage so that we will be aware and accommodate the extra traffic.