Not available anywhere at all (not yet). It’s not hard to make a Roku channel. The sample code from the Roku Developer SDK is quite useful, and all you need is a good idea, a media server, and a little bit of effort. My channel here streams the most recent images from this wordpress blog. I might just make it a public channel at some point – it would be free, of course.
There are all sorts of Roku channels out there – Oregon Beaches, for example. You never know. Almost anything that can be streamed from the web can be streamed to a Roku set top box. It would be a good place for artists to showcase their work. You could even stream your audiobooks that way if you were so inclined (and even have the audio play along with a slideshow).