It’s  really difficult to find video converters that support cross-platform and are open-source as well.

HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded DVD to MPEG-4 converter, available for Mac OS X, Linux and Windows.

It not only accepts DVD sources, but now also accepts practically any type of video as a source and converts it into MPEG-4. The outputs HandBrake support are:

• File format: MP4, MKV, AVI or OGM
• Video: MPEG-4 or H.264 (1 or 2 passes or constant quantizer/rate encoding)
• Audio: AAC, MP3, Vorbis or AC-3 pass-through (supports encoding of several audio tracks)

Other feature includes:

• Chapter selection
• Basic subtitle support (burned into the picture)
• Integrated bitrate calculator
• Picture deinterlacing, cropping and scaling
• Grayscale encoding

Download HandBreak

You can download this open source video converter from here: Download HandBreak – Free DVD to MPEG-4 Converter