Perian, the swiss-army knife for Quicktime, received an update yesterday.
If you have never heard of Perian, then download it NOW! Perian is an open-source (and free) plugin that lets you playback AVI, DIVX, FLV, MPEG, and many other movie file types in QuickTime.
Perian’s latest update, 1.2.2, adds support for WebM/VP8, compressed MKV files and FFv1 lossless video formats. This is also the last version of Perian that will support PowerPC Macs and Macs that run OS X 10.5 (Leopard) and below.