http://www.qnap.com/pro_detail_feature.asp?p_id=67
I know someone who has one of these, and it's a pretty good device.
You can buy a HDD (up to 1TB), put it inside, and then connect the device to your wireless router.
The device will then be available wireless. (Although transfers are slower)
Edit; From what I can remember, it can also be accessed from any where in the world if you have an Internet connection, and act as an FTP/Web server.
Also, you can connect another hard drive directly into the device via it's USB ports, and then transfer files from one disc to another at the speeds of USB instead of wireless.