There are also a few 'mixed marriages' that work. There's a great chart at this website giving you some options.

http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946

I'm building a bike with Campy 8-speed ergos and dérailleurs mated to a shimano 7 speed cassette. The cog spacing on Campy 8 and Shimano 7 cassettes are exactly the same, so if all goes well, the only 'quirk' in my system will be a ghost 8th click.