This is a package of command line tools that allows you to alter, split, join, extract and merge tracks in .mkv files (Matroska). It does include a visual GUI for part of its functionality (merging tracks into a new mkv file).
Download the MKVExtractGUI-2 to get a GUI for extracting/splitting mkv files as well.
File size: 7,687,356 bytes, version: 5.7.0
(Mac OS X (Intel))
File size: 23,535,791 bytes, version: 5.7.0
MKVToolnix was added to MovieCodec.com by Bjarne (08-30-2012).