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.
