Big N is a simple GUI application that uses ffmpeg, mencoder and mkvmerge to convert, compress and merge any video and audio format.

FFmpeg is a free software that produces libraries and programs for handling multimedia data. The most notable parts of FFmpeg are libavcodec, an audio/video codec library used by several other projects, libavformat, an audio/video container mux and demux library, and the ffmpeg command line program for transcoding multimedia files.

MEncoder is a free command line video decoding, encoding and filtering tool and a close sibling to MPlayer and can convert all the formats that MPlayer understands into a variety of compressed and uncompressed formats using different codecs.

MKVmerge takes the input from several media files and joins their streams (all of them or just a selection) into a Matroska file.