At this point I can import almost any avi. to quicktime, but it's a long out of the way process. Also MPG's and some wmv files.
First avi files.
I find that most avi files will play, but have audio problems. The answer is to extract the audio, convert it to an audio file QT likes, and then add it back to the video file. I use DivX Doctor II to extract the audio ( http://doctor.3ivx.com ), What I use next depends on what format it is. If it's MP3 you can use Quicktime.
Quicktime has a problem with MP3s from Windows avi files, but, for some reason, not MP3s on their own. Split em, re-encode the MP3, and put em back together.
If the file is AC-3, then I use mAC3dec.app ( http://sourceforge.net/projects/mac3dec ). It's fast, easy, and you can split an AC3 file into the 6 Dolby 5.1 channels. This is nice now that, to some extent, QT supports 5.1, but means you have 6 files to convert and add to the video file instead of one stereo file.
If the audio is some other format that QT can't handle, I use Sound Studio, Sound Converter, or Cacophony to convert it. ( http://www.versiontracker.com or http://www.download.com )
These have the audio and video ''muxed'' into one file, so while QT can usually play them, it will have problems editing them without losing the audio. bbDEMUX ( http://www.versiontracker.com or http://www.download.com ) can extract the audio to a usable format.
These just got easy, thanks to the new component by Flip4Mac. ( http://www.flip4mac.com/ ) You can open, play and convert most WMV files directly from quicktime, and preview them in the finder.
To put the audio and video files back together using Quicktime, open both files. Go to the video file, and in the pulldown menu select delete track, and get rid of the old audio track. If you don't it's ok, but the file will be much larger when you are done. Now select all and copy, then go to the audio file. Don't paste, which will either make the file twice as long, or replace video with audio. Instead, make sure the playhead is at the begining of the file, and no audio is selected (like when the file is first opened). Pull down to the command ''add to selection and scale''. This will scale the frames per second to make the video exactly the same length as the audio, eliminating ''Godzilla'' effect.
Now you can ''Save As''. Select the ''stand alone'' option so that you can trash the other files. If you don't, every time you open the file, it will need to find the other files before it can play.
You can also ''Export''. This option takes longer, but you can select the codecs that you want, and the size of the finished file.
There are very few videos that can't be made into a Quicktime file with some combination of the above. If Apple could somehow figure out a way to include the technology that the above free/shareware products seem to have no problem with, it would make Quicktime the ''go to'' app that it used to be, and all this hassle would be a memory.
By the way Apple, please take note of the above LINKS next to the NAMES of the needed software. I have found it oddly helpful to others that the links actually go to where the software can be found. Also, the names can make finding the right software strangely simple. At this time I will turn over to you full license of my discovery with no charge, provided you give me credit on every page where it is used, and always call it by it's full scientific name, ''Common Sense''