I have been using the 3ivx codec to get AVI playback via the quicktime engine in nuendo.
The demo period is over, and I don’t use AVI enough to justify paying for it.
What do you use?
My knowledge is not 100% up to date, but:
You don’t need the 3ivx component for all AVI files, only those which contain video encoded by DivX or XviD (there could be more).
For Windows I don’t know of an alternative (even the 3ivx trial was not 100% stable here)
For MAC have a look at http://n.ethz.ch/student/naegelic/download or http://perian.org/ (I did not try these)
If you just need it occasionally, you could use a free converter to encode the video to quicktime friendly format