I saw some cool "bookdarts" on Lifehacker today under the "Things We Like" category. I've been tearing through a bunch of books I got for Christmas this year and have just been using scraps of paper and the like for keeping my place. These looked cool and I thought I'd try making my own.
I was going to use some flashing from out in the garage, but I couldn't find it. Instead I ended up using a pop can as my material source. It worked out nice and the cans cut easily. Make sure you round the corners or you might snag or hurt yourself. Also a pair of needle noses are nice for pressing the folds in them, just don't go too tight because they'll be tough to place and remove.
They aren't as neat 'n tidy as "real" BookDarts and take up a bit of your time, but you can't beat the price and if you fold them the other way you can have nice colored ones from different can types.