I mentioned this in another thread, but it will get buried there.
Pretty cool app for Android and Mac, I got it a few months back for my Droid Bionic.
I was thinking of getting a dedicated metronome for practice off of Amazon, but the one in the app is actually very good.
If not loud enough for you, you can use your phones 1/8" output jack and run it into something louder.
In my case I can run it into the MP3 in on my little Fender Mustang One amp, and it will then be loud enough if I crank up my Carvin V3M some.