This simple tool makes paper creases clean and sharp. I've found it to be indispensable in bookbinding, scrapbooking, and origami projects. I used to use the handle of scissors or a table knife to flatten creases, but this is better. Plus, it takes about half an hour to make if you have some sort of power-sander.
If you don't have any power-sanding equipment:
A) Don't worry...Plain sandpaper works.
B) Check out TechShop! Scads of tools are available to members. That's right, I made this at Techshop.Tools & Materials:
1: Scrap wood 1/4-3/4" thick and no smaller than a credit card (hardwood is best; plywood will work)
2: Sandpaper, 60-220 grit
3: OPTIONAL: orbital, belt, or disc sander (with sandpaper in appropriate grits)
4: OPTIONAL: drill or equivalent; cord or chain