I use this Sena Ultra thin case.
Very nice soft leather, adds minimal amount to the overall size of your Droid, but does give it some protection.
It'll protect it in your pocket/purse from getting scratched from loose change, keys, etc. It'll also give the Droid some protection from a fall, but not as much as other cases, I'd imagine.
However, I like to carry my Droid in my pants pocket, don't want a belt clip, and want to keep it as slim and lightweight as possible.
Not the cheapest solution, but it is very elegant! Droid fits in tight, in fact, the case loosens up after a few days, because when you first get the case, the Droid is a VERY tight fit!
$30 from Sena.