The Red Branch Guild
It was founded March 2005 on the IrishGaming.com Forums and is still going strong. Members use the European Argent Dawn server, which is intended for roleplayers, and play on the Horde side. It has a new website (shared with two other allied guilds).
As part of its Pact with the allied guilds, the Red Branch formed a 25-man raidgroup, COBRA, which was in the top ten Hordeside raid communities/guilds throughout the Wrath expansion, and included one server-first bosskill. COBRA has folded since the launch of Cataclysm, but the Red Branch is running it's own 10-man raids (and is currently actively recruiting, especially for healers). The Red Branch is by no means exclusively a raid guild - members enjoy all playstyles and aspects of the game, including instances, crafting, playing the economy, chasing achievements, and so on.
Anyone interested in joining is asked to post an application on the guild's website at www.arcorb.com.