I can highly recommend CasparCG. At the moment I’m not sure I can classify it as a simple and user friendly CG but for the OP’s request of serving up stills etc. the simple client that comes with the download is enough.
Caspar’s real feature is that it’s so open that you can do whatever you like with it. I recently finished over 40 hours of broadcasts where we had Caspar collect comments from Facebook that the PA ran as a ticker and lower thirds depending on the posts. Caspar was first used for election TV to simplify the data handling, not just on TV but also on the web (where they practically used the same Flash interface).
Caspar is built on a client-server architecture (though you can run both on a single machine) so that you can have one client control various servers and their outputs. Imagine going to a new segment and the projected backgrounds of the set change as the graphics come on screen, or changing just one screen with a competitors score in a game show.
If you have the time to do some digging around Caspar and the demo projects you’ll see what a powerful engine it brings to the game. I for one won’t be spending anymore time on other CG’s.