So somehow I missed that X-Com became free at some point. So now I'm wondering what else I've missed. Is there a maintained list of free games anywhere on the net?

You can always see the current 18 "free" games at the PS+ website (that's for North America, don't ask me about Europe!) 1 or 2 games rotate in/out every week. Anything that's rotated out you can't get anymore, unless you "bought" it when it was "free."

For EU I check the PSblog or you can go tot the PS+ site.