Christopher Roe, a.k.a. Mel Ebbles has released several of his models for free. Look for them in the Models Archive and Fun Stuff sections of his Genet Models website. You can also read more about this in this link.
There are wonderful pieces, including vehicles, mecha, structures and props. Some are intricate but all are worth the effort, and they build perfectly. Here is a build of the TA160 Shrike, in a custom color scheme courtesy of my malfunctioning printer.
This is the small version (1:100 scale), printed at 100% scale. It takes a single page of paper. The built model model is 9.5cm long and 7cm wide. Here is a shot with some 15mm scale minis.
If you are new to papercraft and want to try your hand with some of these models, I would recommend starting with the Runabout Floater or the Itoyo 950 robot. Each of them requires printing a single page and the pieces are mostly boxes (the Itoyo package also includes some crates that are literally boxes, as expected.)