Open source / low-cost videoconferencing options?

I'm working with a small non-profit to help them use technology to reduce costs, and one thing they are looking to do is implement some kind of web videoconferencing to enable remote meetings with a few partners. Skype would probably be fine for the one-to-one meetings, but I'm not sure how to tackle the one-to-many (specifically, one-to-three) conference scenarios.

Wondering if the HN community could advise on some decent low-cost solutions. I think this organization could probably spend a couple grand on a hardware/software solution, if that would even get them anywhere. My assumption is that the remote locations have a decent internet connection and a PC as a minimum baseline.

Thanks, Stacy