Toys powered by cams and cranks, known also as automata toys.
Some of them are very easy to make but others require high level of precision.
Understanding Automata and Motion
Very nice PDF guide about building automata from cardboard and materials on hand. 10 pages, very detailed, with good pictures.
How to make your mini "Crank Shaft"
Obviously you may want the pistons at different positions or more/less pistons.
Go to the guide.
How to Build a Crank and Slider Mechanism
The basics of building automata - how to make crank and slider. This powers most cam and crank toys.
If you haven't yet made any mechanical toys yourself, you can start with this guide.
The Automata Maker Kit
The folks from Cabaret Mechanical Teatre are making great stuff so if we can make similar automata with this kit, I'm all for it.
How To Make a Hatching Cam Toy
This is a simple toy that I made from paper. I don't classify it as papercraft however - it's better to make it from cardboard or wood.
A Simple Cam Mechanism
You can play with it to get a better idea how things work.
A very funny and simple crank toy. There are no templates to print and cut but you can figure the shapes yourself.
Mechanical Boxing Toy
See the paper.
Cowboy Animated Weathervane
Cool detailed PDF plans and templates to build this automata. Grab them free here.