I tend not to use PTT...Continuous/Voice-activity FTW.
But sometimes I have to, then I use Caps Lock. Can be quite annoying, but everything else is bound already.
Skype doesn't have a PTT button.
Voted Other: Footpedal
§ here. Not the best but want a key I won't press by mistake when doing some other stuff. Giving too much orders during raids and that key is really hurting my dps.
1) Noise cancellation microphone
2) Voice activation
3) ...
4) Profit!
Mine's my "fn" function key, bottom left on my macbook pro. Sometimes I mistake it for the control key when I'm looking at armor :P
Middle Mouse Button or Mouse Button 4 (which is the single thumb button I have), depending on the game.
For WoW, and most games, it's Middle Mouse, since I find that it's an uncomfortable button to use a Hotkey.
For a few games that inherently use the MMB (Left 4 Dead for example), I use the thumb button.
I use "z". Control is just too far.
Middle mouse button.
[ < left bracket is my ptt. I'm told that this is weird but at the the time I choose it (after ctrl kept making my vent spaz out and quit working) I was using right bracket for my feedomatic addon so I was used to reaching there.
I use Numpad 5. I stripped an old keyboard of all buttons except num5 and threw it at my feet. Yay for ghetto pedal!