To make Quicktime play anything, install perian.
http://perian.org/
it supports:
AVI, FLV, and MKV file formats
MS-MPEG4 v1 & v2, DivX, 3ivX, H.264, FLV1, FSV1, VP6, H263I, VP3, HuffYUV, FFVHuff, MPEG1 & MPEG2 Video, Fraps, Windows Media Audio v1 & v2, Flash ADPCM, Xiph Vorbis (in Matroska), MPEG Layer II Audio
AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3
Subtitle support for SSA and SRT
I've found this to be the best way of doing things. it means that you get live previews in the finder fr example, and you only ever need to have one media player open.
PS: Thank Fuck apple finally relented and let non-pro quicktime do fullscreen. That was one of the most pig headed things.