Smaller books have the annoying habit of snapping shut if you don't hold the pages down with your hands. Throw together your own book weight with some coins and some duct tape to keep it open at all times.
Book weights are great little tools for keeping books open without the help of your hands -- say, if you're cooking and want to keep the cookbook open to the correct recipe -- but they usually run between $10 and $20, which just seems unnecessary for something you can throw together at home. DIY website Instructables shows us a super easy way to make one right now: just throw some 5c coins between two strips of duct tape.
If you have a bunch of larger books, you may want to follow the design of traditional book weights and stick more coins in the ends of the weight, but it all depends on your specific needs. And, of course, while coloured duct tape and pens might make it look crafty and pretty, you obviously don't need to head to the store for anything -- whatever duct tape and low-value coins you have will do in a pinch. Hit the link to see more.
35-cent Book Weight/Bookmark [Instructables]