It looks like there's a new channel called MyMedia which allows you to share your media from a computer to a Roku. It sounds like it does exactly what the Plex channel does. What's the difference and how do they compare to PlayOn?
Plus, Plex uses a lot of horsepower to transcode HD media and push it to your Roku.
Don't try it with a weak computer. I manage to push 720p with my 2.3 Ghz dual core, but it's running nothing but Plex and PlayOn. I would not attempt it with anything less.