http://tronical.com
I got two for my Fender Lone Star and my Epiphone Paula and this thing just blows my mind each time I use it. HIGHLY recommended, especially at the current reduced price. Because it works like a clip-on tuner you don't even need to have your guitar open, just turn the volume down, strum the strings and your guitar tunes itself. Perfect for on-stage or just in the rehearsal room, one quick strum between the songs and you're safe for the next one. And you can even use it for alternate tunings - no more separate guitar for that Dropped D or Open E tuning!
A quick video that shows what this is about: