How does converting files via your default browser sound? Media Convert does just that. Using a simple drop-down menu based interface, users can upload files directly from their own machines and have the respected file format converted to something new. Convert MP3’s into suitable ring tone formats for your mobile handset. Convert heavy JPG images into something more manageable (such as a .GIF, or .AVI).

Seems completely reliable - based on personal experiences converting generic MP3’s into appropriate ring tone formats for the bargain basement price of free.