Several people have mentioned CGCGCD. I've been using a slight variation on that for years: DGCGCE. I started off with plain open-C (CGCGCE), but that puts the bass F on the 5th fret. That's over the horizon as far as I'm concerned, far too much of a stretch when playing at the 1st position. Raising the 6th string to D solves that, putting the F on the 3rd fret, and bringing several keys - C, G, F, B-flat, E-flat and their relative minors within easy reach. D is a bit of a fiddle, although a 2nd-fret capo solves that when the need arises.

Obviously a bass D in this tuning loses the ring of a deep tonic C, but I thought that sacrifice was worth the increased versatility.

I originally got this tuning from attempting to copy Jimmy Page's acoustic piece 'Bron-Yr-Aur', from Physical Graffiti, although I've since been told that Page has an A-string in there somewhere.

Has anyone else found that these open and modal tunings can be a bit of a drug? It's very easy to make a guitar sound nice without doing anything very much, and I'm sure I've got lazy using DGCGCE. When occasionally reverting to standard tuning these days, I do find it hard work playing things that used to be straightforward when I played in standard tuning most of the time.