This is, by far, one of the coolest online resources I’ve seen for acoustic guitar: www.gtdb.org. Don’t even bother to read this post, just go check it out.
Guitar Tuning Database
According to their homepage, gtdb.org has been going since 2011. The site keeps a huge list of alternate guitar tunings, racking up 507 different tunings. Many of the tunings are user-submitted so, in addition to Open D, you also get some random ones like Standard <2 Inversed (whatever that is).
A quick explanation here: “Alternate” tunings refer to all guitar tunings outside of the normal “Standard” tuning (i.e. EADGBE). Often people speak of “Open” tunings. Open tunings refer to any tunings that create a chord when all the open strings are strummed. Thus, open tunings are a smaller subset of alternate tunings.
Learning how to finger chords in a tuning can be painstaking work. If you click the “chords” link next to any tuning, you get a full list of chord positions.
Scales are also tricky in new keys. Click the “scales” button, though, and you get the full fingerboard layout for all modes in your new tuning.
Note: One drawback is that the familiar CAGED system doesn’t work. You can create some of your own “boxed” positions for your new scales, but it’ll take a little homework.
This part of the site is good, but hit-or-miss. There’s a lot of tabs entered and tagged to the appropriate tunings. Some look promising (like the Michael Hedges tune below). Others are just chords-over-words style. If I was going to dig into a tab here, I would do some homework first and cross-check it against some of the other tab sites.