Take Me To Your Ruler(s)!
One site I find myself visiting more often than I'd expect is PrintableRulers.net. It does what it says in the URL, providing a variety of ready-to-print rulers for various needs.
Sure, I could stretch the truth and say that I used them when I don't want to lug around a heavier plastic/metal ruler, or risk permanent marker on any of my sturdier versions. And, yes, it's handy to have a copy or two stuffed into my gaming supplies. But the truth of the matter is that rulers are useful enough in various tasks to go on journeys around the house; sometimes it's easier just to print a new one than try to figure out where my cork-backed metal straightedge is.
Just make certain to print them at 100% scale!
-- Steven Marsh