I used Prolog in a comparative languages course. The biggest program we did was a map-coloring one (color a map with only four colors so that no bordering items have the same color, given a mapping of things that border each other). I say biggest because we were given the most time with it. I started out like most people in my class trying to hack the language into letting me code a stinking algorithm to color a stinking map. Then I wrote a test function to check if the map was colored and, in a flash of prolog, realized that that was really all I needed to code.
http://c2.com/cgi/wiki?PrologLanguage (via programmingisterrible)