MKVToolnix is a set of tools to create, alter and inspect Matroska files under Linux, other Unices and Windows. They do for Matroska what the OGMtools do for the OGM format and then some. The latest builds now have the ability to create WebM compliant Files. More info »
Haali's Media Splitter comes with a small tool that let's you do similar things. It's GUI is more minimalistic, but it works well as a tag editor.
http://jonthn.free.fr/MKVtoolnix/for a nice relatively up-to-date GUI binary for OSX :D
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