I have a BOSS TU2 Pedal Tuner on my pedalboard and it's 1 of the most useful pedals I've ever bought!
Like the Fender 1 mentioned above, it has 2 outputs on it. 1 of which alows sound to pass through regardless of if the tuner is being used or not, or 1 output that mutes the sound whenever the tuner is switched on.
I find it very useful to have a mute feature on it, as I use it when changing guitars live and it doesn't make a sound :)