WinFF is a very nice video converter for Linux and Windows platform. This is just a quick post for those who are getting this error “Unknown encoder ‘libmp3lame'” when trying to convert a video. If you want to explore the tool head over to their website. They also have a very descriptive and well articulated repository of “HOW TO” documentation here.
Installing the libavcodec-extra package resolved the problem for me. Run the following command in Ubuntu terminal:
$ sudo apt-get install ffmpeg libavcodec-extra-52
I use this tool extensively for converting videos to play on my Android devices (Nexus One and Gtablet :))