For those who haven't tried it yet, OpenRaid lets you create and join cross-realm raids for everything from achievement runs to Siege of Orgrimmar flexible raids. It serves the same purpose as the oQueue addon, but with the added option of scheduling events ahead of time. I've joined quite a few flex raid groups through the site and have had success in most of them, including some Garrosh kills.
OpenRaid has just launched version 2.2, adding some nice features to an already great site. The biggest feature is the new "Quick Raid" tab which lets you see raids happening at this very moment who still need more players. Previously, this type of group forming was done in the chat room, which could get a little hairy when you had thousands of people spamming the groups they're looking for. Chat is still there for those who prefer it. Quick Raids work much like existing OpenRaid events except they aren't planned ahead of time.
Event creators will see a new option for repeatable events, which can automatically invite the same people if you want to do the raid again. Head over to the OpenRaid site to see the rest of the new features.