Originally Posted by Gaborlick
You can pretty much do anything you want.
Connect an external harddrive and play any movie or music files you want. There is a downloadable add on to do so, but it doesn't require a Gold Live Account, just silver (no purchase necessary)
If you have wireless (which if its a new Xbox you probably do) you can connect to other computers on your network and share pictures, watch movies or play music. It is actually pretty fantastic.
The only drawback is the newest movie file format (MKV, not sure if its new actually) but it is Blue Ray burned contect in HD 1080 and I can't get it to play on the xbox through wirelessly connecting to my computer and they don't transfer to external harddrives.
MP4's are a great HD format readable on the Xbox (AVI's are the normal DVD rip)
Just a note. You don't need a wireless connection to stream from you computer. I use a wired network because I want the speeds to be top notch without any hitches.