Flash-based Guitar Tuner
I don't play guitar very well, and only pick up my guitar every few months to have a quick fool around with. Every time I touch it it's out of tune, and I spend longer chasing down a good tuning reference point than I do actually playing.
This looks promising... now to teach myself how to do something other than tune the thing and play scales.
http://www.howtotuneaguitar.org/