Here are some of the best 3gp converters for your mobile (psp and ipod too)
Nokia multimedia converter:
This is a nice video converter for all mobiles that support 3gp . This converter can be downloaded free of cost here. You have to register with the forum to download this software , they also provide you with a keygen for this software for free.
3gp converter (freeware):
The name says it all ,this not only converts for mobiles but also for ipod's , psp ,in 3gp and mp4 format . download it from here . unzip and install
Note: You can download a free unzipper software here