Dr Kevorkian94 is correct, on a Mac you can use Apple's built-in web sharing feature to stream video to the iPad/iPhone/iPod touch. It doesn't require any third-party apps or extensions on either the iPad or the Mac and it DOES work with DRM-protected iTunes Store content. In my hands, it also stream more reliably than any of the third-party apps that run on the iPad (such as AirVideo or StreamToMe). However, the user interface isn't as nice as in the custom-built apps and it does require some setup on the Mac in order to work (you need to create a shared folder and make certain that it links to your iTunes content). It also will not convert video on-th-fly as some of the apps do, the original file must be in a form that is compatible with the iPad.
Overall, if you just want to watch movies that are already in an iPad-compatible format then Mac OS X's web sharing is a good solution. However, it's not so good for short form video or music since it just plays one item and then stops (i.e. no playlist support or continuous -- automatic -- playback from multiple files).