I'm a huge fan of the Boss DB-30 - I've owned three (one lost on a trip; I keep one at my practice space and one at home).
It's got a good, audible tone and works well with headphones for practice (though as noted on another thread, the headphone output is pretty hot and can quickly go to painful.) You can't change the tone which is a strike for some people but I prefer it.
You can use all kinds of accent patterns - from every beat being accented to every 9th beat being accented - and you can do mixed patterns (from 3+2 all the way to 9+8 - eg ONE two three ONE two ONE two three... etc)
It also has various rhythms - quarter, eighth, sixteenth; offbeat eighth & sixteenth; swung eighths, triplets, 3:2 and 2:3 clave. Tempos from 40bpm to 250bpm. Two lights (beat and accented beat), a small clip so you can clip it to something while practicing, very compact, inexpensive and portable.