You were waiting for this one, weren't you? ROLLOVERTIPS controls the tooltip-like message boxes you see when you hover the mouse pointer over objects in the drawing editor. If ROLLOVERTIPS is 1, mousing over your drawing displays a temporary panel showing the object type with its layer, color, and linetype. If, like me, you find this a little intrusive, set its value to 0.
You can get double-duty from this command. Not only can you key it into AutoCAD, but if you have a dog named Tips, you can use it to teach him a neat pet trick! ROLLOVERTIPS is stored in the Windows registry. (And believe it or not — and I'll bet you won't — there's another system variable called NOMUTT! Could that be AutoCAD-ese for BAD DOG TIPS!?)