ohashi-lab is a site dedicated to paper design. Shin Tanaka has used his talents to produce numerous paper designs that you can download, print and make!
Readymechs are free, flatpack toys for you to print and build. They are designed to fit on an 8.5"x11" page and printed with any printer. You'll need double-sided tape, thick matte paper and 10-15 minutes for build time.