I've been toying around with building a two-speed with one cog on the rear and just a front derailer and two or three chainrings on front. Sort of a singlespeed bike with an easy to use bailout gear for hills and lazy days. Probably just run a Singulator on the back in place of the rear derailer. Probably not relavent to this thread though, but the bike above would be great for that project.