I assumed this was for d20 stuff, but some people included others, so I'd say a boolean, all non d20 stuff AND stuff that isn't DnD related. For example, my 1E and 2E DMG's are out and even get flipped through occasionally, but Champions, Paranoia, anything else is collecting dust.
For 3E, Psionic Players Handbook is not collecting dust, but that's because I move it around and use it as a hard writing surface. I've used it once since buying it.