This is indeed a nice tool

This service converts a Flash Video / FLV file (Youtube's movie,etc) to MPEG4 (AVI/MOV/MP4/MP3/3GP) file. The service is using a compressed domain transcoder technology (outline in Japanese). Converts FLV to MPEG4 faster and losslessy than a typical transcoder.
When you submit an url, this service will download and convert to the video format. Then you can download the converted file.

Try it yourself FLV to MPEG4 Conveter engine is now OpenSource! You can download the source code via subversion: SourceForge