I use the Steinway & Sons metronome app on my iphone both for piano and riding. If you don't want to carry your phone around, I can recommend the Seiko Professional series digital metronomes. They are small enough to fit in your breeches or jacket pocket, and give you plenty of options for changing how the beats are emphasized (i.e. time signature).
I use one...and I pick the number that matches my horse's natural, forward gait. I like to hear a "beep" on every footfall in the trot, so it's around 150 for our working trot (around 75 for every other footfall...or in rhythm with my posting). This helps me in the lateral work where my mare tends to get stuck in response my own hips getting stuck - the beat reminds me to let the energy flow in a playful, steady way through my hips (and down the rest of my leg). That's pretty much THE place I use it.
I don't really use it for walk or canter - we don't have many issues there, and I tend to ignore it anyway in those gaits. But, for trot work...I love mine!
I have a cheap clip-on metronome I got from amazon (can't remember brand name). I clip it on the pocket of my jacket or on my belt.