BOSS DB-90 "Dr. Beat":
I got this thing at Christmas and haven't fully explored its possibilities, but I know it does quite a lot of stuff. MIDI in, monitor your guitar (and the sound is great - a big loud fat jazz tone w/my Artcore!), and built-in *
Rhythm Coach* - fine-tunes your timing? (Haven't gone here yet.) Tons of drum beats, has human voice counts, count-ins, on-off w/footswitch... jeez, it does a lot.
DO buy the BOSS adapter ($20). I went through the 9v battery in less than three hours. Prices vary wildly - I could have saved $100 just by checking eBay. I dig this thing - use it every single day.
kj