I've seen other threads on guitar tunings but not quite this question: what guitar tunings do you use and what's the logic behind them? Personally, I tend to learn pieces or tunings by understanding the logic, the mechanics of how they work, which is why I put the question this way. Here's mine for starters:

Tuning: Standard
Why?: Fantastic for jazz chords as most of the strings are tuned a 4th apart, so not too far away for the fingers to stretch. Allows for great complex voicings. Makes you a small orchestra.

Tuning: Drop D
Why?: As standard, but sometimes I want that alternating bass on the D/D, which is much easier than E/E having to fret an E on the D string.

Tuning: Open G (DGDGBD)
Why?: The guitar rings out sympathetically and beautifully when tuned this way, particlarly in the keys of G, D, C and their relative minors. Fast runs are less effort because it is easier to alternate between a high fretted note on one string and an open string to get the required notes, rather than playing consecutive notes on the next available fret of the same string. Beautiful sounds can be made easily with only two fingers right the way up the neck, creating lovely dissonances.   

Tuning: G6 (DGDGBE)
Why?: Same as open G, but the top string remains at E. I started using this because I just found it more practical than open G with the top string at E. You have all the advantages of Open G on all but the top string; and all the advantages of standard on the top 4 strings. Magic!

Tuning: B4 (E.F#.B.F#.B.E) or A4 (DEAEAD)
Why?: I first used B4 then bought another guitar to put permanently in A4, a tone lower with super heavy strings to compensate. In some ways this tuning is limiting, putting you (for A4) in the keys of A or D much of the time. However, melodeons are limiting in the same way and everyone accepts that. The advantages are the 2nd, 3rd and 4th strings used as a simple starting point from which to build. I play some of my most complex pieces in this tuning for that reason. Great harmonics in this tuning. Also note that the tonics for chords I, IV, V are, in A, strings 4, 6, 5.

Tuning: DADEAE
Why?: When a neighbour of mine showed me this I couldn't make sense of it at first. Then I realised the key is the DE in the middle - 2 strings tuned only a tone apart. This means that runs including these 2 strings can ring out - the idea being that, as much as possible, you don't play 2 consecutive notes on the same string. This means you can play less with more effect because the strings are always ringing. You can make some lovely unexpected harmonies and dissonances this way. On rare occasions the top E comes down to D for practical reasons. The keys of D and A (major or minor) are easiest in this tuning, but C, G, E are also perfectly possible.   

I used open D for a while but abandoned it. I've never understood DADGAD (not that I've work on it), so I'd appreciate someone explaining that one.

What about you?