The probably most similiar device is the SqueezeBox by Slim Devices Inc. but its inferioe, because its just a dumb client and requires a smart server. Software is available for NAS though, as well.
They aim more to the high-end audiophile market with very sophisticated DACs.
Not quite up to the challenge of RoKu I think, but still a nice design and anice piece of hardware, if you use their server software.