Good point. But not really a metronome but more a Drum Computer no?
In hardware, I have been using
Yamaha QY-70 (small footprint!) more sequencer actually
BOSS DR-880 (and smaller versions, decent drum computer
http://www.thomann.de/de/boss_dr880_drumcomputer.htm)
Alesis SR-16 (older,
http://www.thomann.de/de/alesis_sr16.htm)
In software, there are many drums VSTi which will do a similar thing (Groove Agent, Addictive Drums, EZ Drummer).
If you are only after a metronome, there are loads of Apps for iOS and Android if you happen to have a smartphone, or even small drum machines (I tried chordbot for Android, not bad at all!!)