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
Ted Fjallman builds Picavets from reused plywood that are very light and durable
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.
For those with access to laser cutters, Myles has made a laser-cut picavet rigging that mounts on a bottle top. His most ingenious innovation is the use of bike chain links as pulleys.