Plex Media Server
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Full user review
"Great alternative to iTunes/ATV"
I have a Roku on one of my older TVs which I couldn't justify replacing just because of HD etc. I did want netflix on this set and missed the feature that AppleTV offers in letting me play movies and music from my Mac. Plex lets me do just that.
The initial setup was a little complicated in the server finding all the songs and movies I directed it to, but it does let you set how often it rescans the library. The Roku didn't automatically find the server. I had to enter the IP address.
Once set up, this worked as advertised and I am able to view my Mac's videos (not from iTunes store) and music library (again, not iTunes bought) on my older TV using a Roku. Bliss.