I have a Mumble server running on one of the home computers. Subject to being comcasted regularly, I am leaving it up mostly for the use of donors and staff. Anyone else gets stuck in the lobby for now, sorry.
Mumble is an open source project that is known for its prudent use of bandwidth. You can read about it on their website and download the current version here. It supports Windows (32/64), Linux, OS X, and iOS. There is also a third party version for Android. Something for everyone.