Free radio network (FRN) is what I currently use. I like the user interface, the audio quality is good and VOX activation is simple to set up. FRN can also use com port keying to activate radio PTT. FRN is the only free program that I know of which lets you use com port keying.
Free radio network is quite an oddball. It's mostly used by PMR446 and CB users, and there is a well established network of gateway's all over the world. To use this site you have to register as a PC user or gateway owner, and once you are up and running, you show on the system as being "online".
Mumble is something we have used in the past, and the VOX activation (although complicated) is VERY sensitive. The user interface is not so nice as Teamspeak, but it is workable once you get used to it.
Skype is very well known already, and has an audio quality which has to be heard to be believed. Skype can be used to answer incoming calls automatically and can even forward unanswered calls to your mobile phone! It really is a good tool for linking radio's and is something that I am still playing around with.
Teamspeak is easy to use, administer and modify. Simple but effective.