While there are many other applications which provide similar functionality, such as Boxee, Plex has a very slick interface and great support across a variety of platforms. Plex began as a development fork of the Xbox Media Center, and is now supported by a corporation behind the development which includes a freeware/open-source development community as well as some sections which are proprietary and closed-sourced.
Plex provides a great "10 foot experience" when viewed on a television. The interface for movies, TV and music includes information gathered from the Internet to provide a detailed summary and graphics to identify your selection.
Plex is also a great way to browse through your collections, with a "CoverFlow"-like view as well as search capabilities. In some iplementations, samples of the music being browsed, or even the theme song of television shows are played from the browser interface - very cool!
Among the choices for Roku to stream your own media, I haven't found anything that matches the ease of use of Plex. Plex has excellent transcoding capabilities as well as support of many codecs natively - I have been able to play every file format, from AVIs and MP3s to MKVs and AACs. Rewind, Fast Forward and Resume all work properly. Plex can be added to Roku as a channel, which will then connect to your server.
Plex's aim is to be available on every conceivable platform, and it seems like they are well on their way, hitting many of the big players. With the pace of development, already impressive progress and availability on many devices, Plex is already on its way to being a big winner in this space.
Download Plex for free for all of your devices and give it a shot: