Although not technically an "effect" pedal, a pedal-based tuner has become an indispensable part of many guitar players' pedalboards and live setups. It goes without saying that playing in tune is a crucial part of any live or studio performance, and it's just not practical in many situations to use traditional handheld guitar tuners.
Once you realize a tuner pedal is a necessity, then you need to decide what kind to purchase. There are several different models available. Some pedals emulate traditional handheld pedals, using LEDs to indicate whether the instrument is sharp or flat. Other pedals use either strobe or virtual strobe technology. While more precise, strobe/virtual strobe tuners also take a bit longer to use. There are even some pedals that allow you to tune more than one note at a time.
Finally, you also want to make sure that you use a pedal that doesn't color your sound or add unwanted noise, though that's not as much an issue these days, given the quality of such pedals.