Sorry, I couldn't resist the temptation to be overly dramatic.
I tuned my 5th string down, in G tuning, from GDGBD to get DDGBD. It makes an interesting variation for some tunes, e.g. "Bound To Ride". D, of course, is part of the D and G triads, and is part of C-add-9 chord, so the re-tuned 5th string's quite harmonious with the G, C, and D chords. At least I think it sounds good.
