Mumble voice chat is my favorite. It is free, low latency, has no ads or popups, small install, and runs stably. Mumble runs in the background while i play a game, and puts a small indicator on my screen (adjustable) so i can see who is talking as they key the mic. You can whisper to private parties while others are speaking. You can add sub-channels and edit admin rights. Mumble is apparently very popular amongst FOnliners.
Ventrilo is another good one with strong features like Mumble. I've had good success with it too.
RaidCall? I know people use it, but i never tried it.
Skype? I consider this to be a highly effective spy program, worse than a virus. I will never again install it on my own computers. It remains however very popular and it has excellent video functionality in addition to voice.
For IRC text chat only, i use
ChatZilla but i do hate how it is advertised on by a group called "Hackers Are Us". Not so great there.