It's not the first media hard drive we've seen, but TrekStor's new MovieStation gets kudos for being one of the biggest. The MovieStation stores up to 500GB worth of media and can connect straight to your TV (pumping out video via component or composite connections and audio via optical/analog). On the video front, the drive supports all of the popular formats from MPEG to DivX. It'll even play your HD content. Audio-wise, there's no AAC support, but you do get MP3, WAV, WMA, and OGG. Like other media drives, it also comes with a remote control. Pricing starts at $299 for 250GB and $399 for 500GB.
TVIX Box M-3100U plays most nearly all formats on nearly any TV, has a remote and is only limited by the size of the HDD you can find. It will also transfer or stream from external devices via 2 USB host ports. It is also a proven product with good support. Only downside, no network. Instead it plugs into your computer via USB like any external HDD, you transfer your media to it, then walk it to your PC - can't beat it for $169.
TVIX Box HD M-4000PA supports High-definition up to 1080i. With an onboard LAN port and optional wireless you can transfer files to and from it via FTP, or stream directly to your TV from shares on your PC(s). It will even stream SHOUTcast Internet radio directly off the web. With the T410 add-on it becomes a full-on High-Def digital recorder and HDTV tuner. It's $250. Need I say more?
Lastly, the TVIX Box HD M-4100SH adds HDMI output, H.264 decoding, SATA HDD support and full 1080P resolution for $359 (yes, it's a bit pricey).