I've heard that skype is easy to hack, so I only use it with people I trust completely.
I've heard that there are a few free options that are good, but I haven't tried them. I have had good experiences with both Ventrilo and Mumble. Mumble was a bit more confusing for some people when they tried to set it up, but I liked the quality. It also adjusts everyone's volume, so you don't have to go in and change them individually, which is a nice benefit.






