The default media player for Series 60 3rd edition is already quite good compared to the previous versions (e.g: it has fullscreen mode, better streaming, etc), but sometimes this is not enough. The file size vs quality of real media files is not the best, for some people, DivX is better.
Currently DivX network provides freeware beta version (time limited) of their product at: http://mobile.divx.com/login/. You can use many freeware tools to convert your movies to DivX format for viewing movies on the go. It many not beat iPod video, but if you have a large screen series 60 device (such as Nokia E61), then I think that will be good enough.