At some point each year Popular Mechanics names the winners of what they call their “Backyard Genius” awards. They tend to be DIY engineering projects that sometimes are carried out on a pretty impressive scale. 2010’s awards were announced in September, and Jeremy Reid is included for his homemade roller coaster:
A 16-foot drop propels the car to 18 mph; the rider then zips over another hill, down the sloping backyard and around a 50-degree bank that pulls 2 g’s. Nearly 1 minute and 450 feet after the initial drop, the car returns to the lift. Reid estimates he spent $10,000 on the project—though it paid off by helping him lock down a post-college job with coaster design company Arrow Dynamics (now S&S Arrow).
The other Backyard Geniuses can be found here, and they include a guy who built a tandem unicycle.