First of all, let me state I'm in no way an expert on Alternate Tunings...or for that matter, much of anything.
However, I've seen too many examples here that are either not in the correct tuning, or, if they are, too many responses asking, "What is XYZ tuning?"
As I said in the Summary, there are TONS of alternate tunings. Most people are familiar with Drop D and tuning down a half or whole step. You may have even heard of "DADGAD" (otherwise known as "Modal D Tuning), or even Double-Drop D (DADGBD).
What if I told you there are about SIXTY alternate tunings? You'd probably have the same reaction I did...
I won't list all of these, but I will list some. Further, I'll point you in the right direction to find more complete explanations, as well as a host of other tunings...for FREE. ;)
Of course, there are some very good publications commercially available. But, why pay for something that's available at no charge? If you find one that's better than what I've found, or any other source, please let me know.
Before we delve into alternate tunings, let's look the progression I've seen in Standard Tuning to, "Time For Me to Fly," by REO Speedwagon. Great song, btw.
D G A D
Sounds pretty good, right? Now, listen to the recording. Correct key, but the guitar sounds much fuller and richer, right? So, how'd he do it?
He was tuned to Open D...a variant of DADGAD tuning. Open D's tuning is DADF#AD. Using your tuner, lower your 6th (low E), 3rd, 2nd, and 1st strings to D, F#, A, and D respectively.
Are the notes of the tuning familiar? They should be; they're the notes of the Dmaj triad (D, F#, A). Strum all the open strings and you're playing a 6 string Dmaj chord. The six string chord (and lower octave) adds a lot of fullness and depth.
OK...so what about the rest of the song? You tried fretting the other chords and they sounded terrible, right? Since you've changed the tuning, now you have to build the chords. Further, the G is really a Gadd9 and the A is an Aadd4.
Yet, like the open Dmaj, these are simple:
Add one more chord that closes out the phrases in the verses, Gadd9/D, and you have the entire song. Again, the tuning makes it simple:
Now compare the full sound you get with Open D compared to Standard...Open D sounds much better...full and rich. The open treble strings serve as "drones" common to all the chords.
Here are a few other common tunings:
Open C: CGCGCE (you guessed it, strumming the open strings produces a Cmaj chord)
Open G: DGDGBD (ditto for a Gmaj)
Open A: EAC#EAE (ditto for an Amaj)
Here's a link to a free 96 page PDF written by Bill Sethares. He explores these, as well as over 30 other tunings in great detail. He also includes chords for each tuning, as well as instructions on how to find chords in each tuning.
Here's a link to the Sound Thinking Chord Finder. It includes about 60 alternate tunings, tunings for various instruments, an interactive chord finder, and many other neat features. Again, it's FREE!
As you can see, one lesson covering all of these tunings would be REALLY LONG. By using a practical example, I think most people can see the value of exploring alternate tunings on their own. Besides, you won't learn this by reading...you'll learn by trying these tunings and applying them.
Hope this helps!