How to transfer a movie (.avi) from pc to iPad without iTunes?
There are a lot of alternatives for iTunes which will allow you to manage your iDevice. 
Floola, Songbird and Yamipod are three popular alternatives for iTunes. 

Then there is software which will allow you to treat your iDevice like a external hard drive, like iTransfer or SyncPod (not for Lion or iOS5). 
But my overall favorite is Bigasoft iPad 2 Transfer. I've used it quite some time and I have no complaints about it.

If it's an option, you could try and stream it. Make an server out of your Mac and stream it with Air Video to your iPad.

And there is even an alternative by using the cloud. Dropbox is a great example of this. It's all listed in this article of MacWorld, which made me wonder if you even have searched far and wide enough :)