Damn good question. I guess you know you've got them all when you no longer have problems.
This is why I hate AVIs and other silly files like oggs, rars, etc. Why not just use ZIP and MP3 and AIFF (or FLAC) -- in other words, use some sort of standardization. Oh well, to each his own.
I looked in my Library and found the following:
3ivx D4
AppleIntermediateCodec
Divx 5
Flip4Mac (from Microsoft -- lets QT play windows media files)
oggImport
Save as AVI
Theora (have no idea what this is)
Xvid Codec
These seem to be all I need. If you are missing any you should be able to search for them via Google.
Of course, I also have QTPro instead of the regular free QT -- though I don't know if this matters.