Otherwise known as 'Vent', Ventrilo is our in game voice chat application of choice. Check the guild tab in game to find the connection settings for our server.
Deadly Boss Mods is an invaluable collection of addons for each Boss/Raid encounter. DBM for short, also includes helpful addons for PvP and battlegrounds.
Omen is a threat meter. Basically, enemies in WoW decide who to attack by deciding who is the most threatening based on the abilities you use. What Omen does is provide accurate values of your group's relative threat level on individual enemies, so that you can see when you're in danger of pulling aggro (or, if you're next on the snack list if your tank bites it). This info is usually only critical in raids, where only tanks can survive aggro, but it's useful for any multi-player situation.
If you are looking for more addons check out Curse.