Netgear ReadyNAS Duo 500Gb Network Storage
This model comes with one 500Gb drive and space for another.
More than just an external hard drive - a NAS (network attached storage) attaches to your router by ethernet and has a myriad of functions:
Sharing files & and a printer to multiple computers (Mac & Windows).
Backing up computers on the network.
Central iTunes and other media.
Bittorrent straight onto the NAS (don't need a computer on).
External drives and USB sticks can connect to the NAS and transfer data.
2 drives to mirror data in case of drive death.
There are other things too.
Quite a few of these around, this is one of the cheaper/more versatile ones. Great if you are a multi computer household/office.