The Picavet rig has a long history-- developed in the 19th century for kite aerial photography and rediscovered in the 1980's. It uses a continuous loop of string through four points on a platform and 2 or 4 points above to automatically level a platform. See this great page on Kaper.us for construction details. Quality commercial kits are available from Brooks Leffler.

Several mappers have built simple variations of the Picavet:

Chris Fastie built a dual-camera Picavet rig and has a great video showing its construction and first flight.

Michele Tobais uses a Picavet rig built on a trash can to provide extra protection for huge cameras

Jessi Breen is developing a mini-cd crate picavet to protect her camera

The FABA collective has made a picavet similar to Michele's trash can rig but with a few ingenious uses of rubberbands and springs. Definitely worth integrating into your kit.

Mathew Lippincott has a simple modification of the PET bottle rig to add a picavet rigging