I have found the wittner portable quartz metronomes to be very useful. They are great for traveling because they fit just about anywhere, in a guitar case, sax case, etc. They have many features which i find helpful when practicing, such as variable beat and rhythm setting, and aural as well as visual LED beats.
Most of them come with an earphone in case you are playing your instrument louder than the sound of the metronome. See Wittner Quartz Metronomes at Pianoscomplete.com