There's also a program out there called Graph Edit, which will actually read the headers of a file and determine what playback filters are necessary. Once you get the file open you can remove the "Direct Show Video" and "Direct Show Audio" outputs and swap in file writers for any format you have the codec for. (DivX, Xvid, whatever) It takes a little while to figure out and the compatibility with DivX-based codecs is minimal (it encodes at max everything) but it handles almost every file type you can throw at it. The only filetype I haven't been able to convert with it is RealPlayer files.
By the way, any RealPlayer -> anything else converters out there?