AVIs play fine in Quicktime 6.5
As long as they have AVI standard audio or MP3/AC3/Ogg/AAC as for those there are codecs out. The latest DivX.com codec has an importer feature that fixes the MP3 problem and with the AC3 component also allows AVI playback with AC3 audio. If your video is encoded with Xvid you might either want to use DivXTool to change the FOURCC from Xvid to DivX or also get the Xvid.component that will add Xvid functionality to the DivX codec. I actually have the 3ivX codec installed together with the DivX codec as the decoder of the 3ivX codec is faster and I can still use the AVI Importer feature from the DivX.com codec. This combination of codecs allows playback of the following formats:
Video
DivX 3.11, 4, 5
Xvid
MS-MPEG4.4 (FOURCC MP43)
3ivX
with these audio for AC3 and MP3 works just fine, AAC audio as well btw.
Problems are still with files encoded with MS-MPEG4.1, MS-MPEG4.2 and MS-MPEG4.3 (FOURCCs MPG4, MP41, MP42) These can be played back with the FFusion codec, however the audio will not work like that so you might either want to use DivXDoctor to convert the files to MOV (faster) or reencode them as DivX with audio passthrough (slower but then the files will also work on hardware DivX players, like the KiSS DVD Player).
There are problems with DivX AVI files that use closed source Audio codecs such as WMA8 and WMA9, ask Microsoft if they finally get their asses up and write Quicktime codecs for their crap..... For WMA8 there is a converter out (DivXWMAConverter) that will convert the audio into MP3 for WMA9 it is in the works, it depends on how quickly the FFmpeg and mplayer folks can hack the MS codecs to work without having to use Win32 DLLs. In the meantime flame anyone who uses those ****ty codecs in the first place.
On a sidenote, there is also no Indeo Video codec for OSX. Ligos (the company that makes Indeo) and Apple both point fingers at each other and say the other is to blame for that, so I guess it will never happen that there will be an OSX codec.
VLC and MPlayer of course will playback AVIs without much hassle and especially VLC 7.1 also has a greatly improved support for OGM and MKV video files, I however prefer to convert all my files to standard conforming AVIs (meaning DivX codec support DivX video and MP3 audio) simply because that way they will also work in my KiSS DVD Player.
Cheers,
Ahmed