It's pretty easy to do this by hand, actually. We did it all the time when we were kids and we'd forgotten to staple the pages of our reports together.

Grab a couple of pieces of paper at the corner and fold the pages down together, dog-ear style. Then make two small parallel tears on the fold (perpendicular to the folded edge), creating a tab. Fold the tab down and you're all set.

It actually holds pretty decently, too.