I've used some of the wireless VOX, voice activated, systems with mixed results.
You have to make some sort of a noise before you start to talk, like a "ggg" sound, otherwise the first part of your sentence gets cut off. It also didn't work very well for some women with higher pitched voices, they couldn't get the unit to turn on when they spoke!:head
The hardwire and push to talk systems seem to work out a bit better. They limit your mobility a little bit more because you either are attached to a wire, or you have to manually push the button to speak.
In a pinch I have also just put my regulator second stage right on my buddies head and yelled at them.

This works out best if they have no hood on and it is actually quite effective in getting a message across. It's very important to make sure that your second stage is actually touching their skull to get the sound transmitted most clearly.
I would think that rebreathers would work quite well for talking to eachother underwater without any extra electronics on them at all. It wouldn't give you surface contact, but you could certainly talk to your buddy without much of a problem.
Jon