I use Ventrilo and Team speak, although I prefer Teamspeak because it seems more user friendly. If I need to talk to one person directly for non gaming related matters, i will use Skype, but its often easier to just call the person with an actual phone.  Some of these are more for just talking to people rather than gaming but they can all be used for both.
 
What do you like the best?

For just chatting with friends, Skype. For playing together with a team/guild, Ventrilo.

" For just chatting with friends, Skype. For playing together with a team/guild, Ventrilo. "

Teamspeak 3 is the shit

I downloaded the ventrillo server program to run my own free 8 person ventrillo  server to chat to mates. other than that sometimes steam chat a little bit 

Mumble is pretty awesome.

I've been having decent luck with Teamspeak 3.