Boss DB-12 Dr. Beat. It's got tempo, rhythm, beat, note, pitch, timer and a stop watch. All of that in a small little pocket size thing-a-ma-jig. Really cool.
If by "gradually" you mean it automatically speeds up at a particular rate - I'm not sure why you would want this. When practicing, you want to make sure your sense of time is right on, regardless of the tempo setting. Then, selecting a quicker setting will mean something to you, in addition to helping you develop technique. The best value out there in metronomes IMHO is the inexpensive Qwik-Time. It should be approx. $20 in the US. Sounds better than most of the more expensive electronic models - almost a real wood block sound, with a visual indicator (LED) as well as infinitely variable tempo settings.
No substitute for the real thing. I've always thought it's easier to "feel" the beat with a *good* drummer than a metronome. Still gotta have a metro though
if you practice near your 'puter, you can go to Online Metronome. I don't know how good this one is but you can download it for free, metronome. if you do a google search you can find a bunch of free downloadable metronomes. i have one that you can change the sounds and it has it's own visuals, but i can't remember where i got it from. i can check when i get home and see if i can find it. if i do, i will post the link.
I got the advice from a drum teacher to absolutely not play exercises at gradually increasing speed. Play everything at a set speed, stop playing, and then start at a higher speed if you want to. This way you train yourself to keep a specific tempo, not slide up or down.
A good drummer is just using a metronome by proxy. (Cause if he never used one, he probably isn't a good drummer) I have a little cheapie - which is almost great BUT - WHY does everybody feel like they need to put beat accentuators in metronomes nowadays? The one I have does, you can't turn it off AND they got it wrong. The Downbeat (actually just the one) is pitched HIGHER than the others. It drives me absolutely crazy. I want a slappernome though. A metronome with a tap tempo function that fits in your pocket and thumps rather (or in addition to) clicks. Stick it in your pocket, slap your leg a couple times, and away it thumps. You could still hear it when playing loud. Maybe it should even have electrodes and apply electric shock when you rush or drag.