Mac: If you chat from your computer a lot, then you know how much of a pain it is to mute your microphone quickly. Push to Talk is an app that adds a simple hotkey mic toggle so that unexpectedly loud noises don't get through.
As the name suggests, Push to Talk works by adding a hotkey switch for your microphone. When you hold down the hotkey, your microphone isn't muted. Let go of the key, and it's muted. You can customise Push to Talk to simply toggle the mic on and off as well. Either way, it's a simple but incredibly useful utility if you work in a loud environment and do a lot of voice chat.