Someone already mentioned Prolog, and I agree it's good to "learn" totally different ways of doing things, even if you never use that language to write real products.

Taking that to more of an extreme, check out Intercal, a language whose whole charter is to do things differently than any other language. It was supposed to be a joke, but why do the concepts seem to be less than funny today? The way conditional execution is performed by turning off specific lines, rather than jumping over them, sounds a lot like Intel's new 64-bit chip. The way you can disable specific commands sounds a lot like "sandbox" security environments as seen both in Perl and Java. It goes on and on.

Read it and get a good laugh. But read deeper and see that the way we do things is really a very narrow, even provincial, viewpoint.

—John