+1 for Plex. I'm a lifer.
Plex works as two primary pieces of software: A back end server and a front end client, just like XBMC if you have ever used that (it actually began as an XBMC fork). You have to install the server on your Mac that has access to the files you want to stream. Then you install the front end client(s) on whichever devices you want to stream to - Nexus Player, Roku, Android/iOS phones and tablets, laptops or Chromebooks, etc.
For the server, you basically install the server software on your Mac and point it to the folders you want to include in your library. This can include your iTunes folder, however my understanding is that any iTunes DRM files will not play because of the restrictions iTunes put on them. Torrented files work just great, even if added to iTunes and accessed in Plex from the iTunes folder on your Mac. I used to put them in iTunes as well, but have since stopped nearly all use of iTunes.
Once you have the server set up and pointed to your media file directories, you then have to install the clients. There is a Plex Home Theater app for Mac; do not confuse this with the server app, they are two different things. Even if you only want to view the movies on your Mac, you still need to install the server and the PHT client. On mobile, Android and iOS each have Plex apps that work great. Android TV has a Plex app as well.
To take things a step farther, I also have an Apple TV and before I got a Nexus Player, my method for using Plex on my living room TV was this: I have a Logitech Harmony remote with hub that has Bluetooth. I set that up to be able to control my Mac Mini in the office. I added a Mac app to the Mini called Remote Buddy that basically allowed me to map the Harmony remote buttons to control various apps on the Mini, including Plex. Then I would use the new Airplay screen mirroring feature added to OS X in Mavericks I believe to Airplay my screen to the ATV. Vioala... Plex Home Theater on the Mac in my office Airplayed to the living room TV and controlled from the couch via a Harmony remote. I love it because I find the PHT interface much nicer than any Android TV or mobile device interface. I do still use the Nexus Player app from time to time though, but would like to see some improvements to it.