I usually don’t plug products, but the just-released PolyTune app for the iPhone is amazing. You strum the open strings on your guitar, and the six pairs of dots—one for each string—show which strings are out of tune. Instead of going through each string one-by-one, you can see immediately where the problem is.
In the image at right, the 4th string’s a little flat, and the 2nd string’s pretty sharp.
I just tested the app on my acoustic guitar, and it seems as accurate as my favorite “normal” tuner.
The first 25,000 downloads of the app are free, so let the feeding frenzy begin. More info here: http://www.tcelectronic.com/polytune-iphone-app.asp