Vent for guild runs. My IRL buddy has a mumble server so I hang out in there a lot. And Skype for PvP. --- Even addiction has diminishing returns. Johnrockks#1547 http://us.battle.net/wow/en/character/kelthuzad/Marisette/advanced
Axon is great for the 3-D positioning if that's something you care about. The disadvantage is that you have to do a signup and a bunch of jazz to get to use it.
Vent is great because the steps to using it are: DL Client => Info => You're in. It's also extremely lightweight.
Vent lacks volume normalization which is something some other services have, but I'm fairly sure that's going to use more resources. You can manually adjust the volume of other players under the miscellaneous special effects menu anyway which will take care of everyone in your guild. The only problem is if you post info for a PUG achievement run or whatever you're going to have like 2 mice and 2 elephants for every 10 people.
Mumble is open source and I know a lot of people are all "rah rah open source" so whatever. --- "For what is a man profited, if he shall gain the whole world, and lose his own soul? ..." - Matthew 16:26