Eiffel

From Wiki

Eiffel is a programming language that I find abstractly appealing, but it never became as popular as C/C++/Java/etc and so its user community and tool base are small.

Its main advantage as I see it is the support for Design by Contract : function preconditions and postconditions (including the "old" keyword to refer to the state at the start of the function call), class invariants, loop invariants, inherited conditions, and general asserts. Some other languages like D claim to support DbC to varying degrees but seem to usually miss the mark or only pay lip service to DbC.

http://eiffel.com is the company of Bertrand Meyer (Eiffel's creator).

http://se.ethz.ch/~meyer/publications/online/eiffel/basic.html is a brief intro

http://smarteiffel.loria.fr is SmartEiffel (formerly SmallEiffel) an open source Eiffel compiler