iMac as video server for iPad
You can use iTunes Home Sharing. 

For it to work, you need two thing:


All your movies in mp4 for iTunes (Ideally with Metadata and Images, looks cleaner and is easier to search on the iPad)
An iTunes Store Account


You then put all the movies in iTunes, leave it open and activate "Home Sharing" in iTunes->Preferences->Sharing



Check "Share my library on my local network"

Then, go on your iPad, make sure you are connected with the same iTunes ID and go in the standard Video app. You shall see you library that can be shared. Tap it, let it load and there you go, you have access to movies from your iPad! (it also include TV Series)