Something else to mention is Connected Realms. Essentially these are realm mergers only without the problem of name clashes. They're being gradually rolled out on low and medium population realms.
You can also form cross-realm groups. Most PuG raids and world bosses are done this way, using either openraid.us, the Oqueue addon or Blizzard's own raid browser (which is really hard to find, here's a small addon to make a button for it in place of Blizzard's new cash-shop button: http://www.curse.com/addons/wow/bliz...ng-other-raids)