You can move your iTunes library to an external hard drive or a network drive, but you would still need to use iTunes to feed the ATV. You can't connect an external drive directly to an ATV, and all ATV files, whether synced or streamed, have to go through an iTunes library hosted on a computer.
You can try a product called aTVFlash. It's expensive, though, and it will add features to your Apple TV that are not supported by Apple. It seems to offer functionality the Apple TV should have had in the beginning.