Most interesting non-mainstream language?

I would suggest having a look at Erlang - it's been getting a bit of press recently, so some of the learning resources are excellent. If you've used OO and/or procedural languages, Erlang will definitely bend your mind in new and exciting ways.

Erlang is a pure functional language, with ground-up support for concurrent, distributed and fault-tolerant programs. It has a number of interesting features, including the fact that variables aren't really variables at all - they cannot be changed once declared, and are in fact better understood as a form of pattern.

There is some talk around the blogosphere about building on top of the Erlang platform (OTP) and machine support for other languages like Ruby - Erlang would then become a kind of virtual machine for running concurrent apps, which would be a pretty exciting possibility.

Boo targets the .NET framework and is open source. Inspired by Python.

I find constraint languages interesting, but it is hard to know what constitutes forgotten or unknown. Here are some languages I know about (this is certainly not an exhaustive list of any kind):

• Ciao, YAP, SWI-Prolog, and GNU Prolog are all Prolog implementations. I think they are all open source. Ciao, gnu prolog, and probably the others also, as is common in Prolog implementations, support other constraint types. Integer programming for example.
• Mozart and Mercury are both, as I understand it, alternative logic programming languages.
• Alice is more in the ML family, but supports constraint programming using the GECODE C++ library.

Drifting a little bit off topic....

• Maude is an interesting term rewrite language.
• HOL and COQ are both mechanized proof systems which are commonly used in the languages community.

Lambda-the-Ultimate is a good place to talk about and learn more about programming languages.