From Jacek Pazera:
Free 3GP to AVI Converter is an absolutely free application which convert mobile phone video files (3GP and 3G2) to AVI or MPG (MPEG-1, MPEG-2) format.
3GP / 3G2 files are commonly used in 3G mobile phones, but they are not supported by many players and video editing programs. To play or edit 3GP/3G2 video you need to convert that file into more popular format, such as AVI or MPG.
At default settings, the application converts files using Xvid video encoder and MP3 audio encoder, and automatically selects all the encoding parameters so as to preserve the original video and audio quality. Most portable and fixed players should have no problems playing AVI files created with these settings.
The application interface is a very simple and user-friendly. To convert mobile phone video files, just drag & drop them into main window and click the CONVERT button.
Advanced users can set many parameters used by encoder: audio and video codec, audio and video bitrate, frames/sec., sampling frequency, resolution, volume and other.