Media Streaming devices are pretty common these days, but devices that can stream high definition video are less common. This one looks interesting: the Ziova ClearStream CS510 can play back high def Windows Media and MPEG4 files in 720p resolution and has a HDMI port to connect to a HDTV. It connects either to a PC over a wired or wireless network, or directly to a USB 2.0 hard drive. Ziova also offer a version with a built-in DVD drive if you can't be bothered to rip your DVDs.
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Ziova CS510 [Ziovia]