Whether you're a curious developer or a user waiting to get their hands on the Yosemite public beta, check out Ars Technica's guide on creating a hard drive partition. Keep your main computer usable and test all the things.
Apple 21.5-inch iMac/1.4GHz (Mid 2014)
Apple 11-inch MacBook Air/1.4GHz 256GB (Mid 2014)
Apple 11-inch MacBook Air/1.4GHz 128GB (Mid 2014)
Apple Mac Pro Quad-Core/3.7GHz (Late 2013)