Now, truly, no one has an excuse for playing an out-of-tune guitar. howtotuneaguitar.org features a Web-based interface for tuning, and step-by-step instructions in case you’re really a newcomer. The tuner itself goes well beyond the basics, with support for Standard, Drop D, Open C, Open G, Open D, Open G, Half Step Down, Full Step Down, Open E or Admiral tunings. Powered by Flash, you can trigger looped recordings of the strings one by one.
The site has chords, scales, lessons, and a forum; you can even see blog-style entries, like one dedicated exclusively to the “D suspended-four” chord. Is the day coming soon when chords themselves will have their own blogs? (phyrigianmode.blogspot.com?)
Guitar Tuner @ HowToTuneAGuitar