There are a few decent VoIP programs. All of which have up sides and downsides.
Ventrillo - Free up to 8 people, but you need to have an account for more than that
Skype - its Skype ... that should be enough
Voxli - Free Web-based, can lag games
VoIP tends to be good for some people and annoying to others (me). Unless you are doing something that requires major coordination, there usually isn't a reason to have one.
Just my 2 cents.
