I have had my Roku box for a little over a year now, I mainly use it to connect to Amazon Prime Instant Video, Netflix, and occasionally Pandora. I have always known there is more to do with a Roku but never took the time to explore.
Plex took 5 minutes to install on a machine at home, and less time to configure and point to my media library. For the most part Plex figured out what was what and displayed the right metadata, on a few items I had to perform some file name cleanup to get them to display properly.
Next up install the iPad app (or Android), and the Plex Channel on the Roku box and like magic I can access my library from anywhere. The performance on both the TV and the iPad has been excellent so far.
Plex also offers Premium features, which I have not tried, though the upcoming Dropbox integration looks interesting.
Plex Media Server (Available for Windows, Mac and Linux)