Seiko's DM71 Pocket-Size Metronome is a fully-functioning piece of musical hardware in a very portable package that you can just slip into your stick bag or backpack at the end of rehearsal. All sounds are clear and easy to hear, and all visuals are easy to read thanks to the high-contrast LCD display.
You can practice with tempo ranges from 30-250 bpm, with seven different beat accent options to help remind you where one is in many different time signatures. You can also choose from several different rhythm accents for further detail: 8th notes, 8th note upbeats, triplets, shuffle, 16th notes, and 16th note shuffle.
If you need some reference tones (when tuning timpani, for example), you can select from C4-B4 tuned to A=440Hz.
A headphone jack on the side allows you to practice and still hear the metronome in really loud environments, or if you just don't want other people to hear that constant beeping.