Probably the best iPad app so far: Air Video
Air Video streams video from your iMac to your iPad (and iPhone). You don't need to copy your videos to the device just to watch them. The amazing thing is that you don't have to convert them to mp4, air video will convert them on the fly and the video will start almost immediately. Stremaning is done in a 2400 bit rate so you really get good quality. At only $2.99 for the pro version it is a bargain.This is awesome!