3 apps I find myself using all the time:
TeamViewer - Lets you control your computer from your iPad / iPhone (also works over 3G). The free version is full-featured, you just have to agree not to use it for commercial purposes.
AirVideo - Converts just about any video format on your computer and streams it to the iPad / iPhone (also works over 3G)
Skyfire - Converts Flash video on the fly. Because
South Park Studios | Kenny, Cartman, Stan & Kyle | Watch Full Episodes, Clips & More rules on iPad.
If you really want to blow your mind with your iPad, throw down $99 on one of the new Apple TVs. Get the Apple Remote app and you can use your iPad to be the ultimate remote control for a killer media center. With iOS 4.3, you can stream video from YouTube, etc. straight to the Apple TV using AirPlay. Also with AirPlay, you can stream any format video to the Apple TV via the AirVideo app too. It's pretty awesome.
(insert long, drawn out "nerd slurp" here)