C - Some people describe this as portable assembly. They are right. C feels remarkably close to the ideal minimal abstraction from the concerns of computer architecture. I can't think of a good programmer who dosn't know C well, there is a good reason for this.
Python - Possibly the most pleasant language I know. Also by far the easiest language I know of to teach to absolute beginners. Python has the advantage of supporting both imperative, functional, and object-oriented paradigms. However it's best feature is its truely remarkable community. I am unaware of any community that has such a healthy focus on usability, readability, and statistics-driven optimisation. I don't do much python programming anymore, but I still enjoy reading the community.
Scheme - Take one very powerful language; add very simple syntax; mixin safe syntactic closure and you get scheme. Hence it is widely used in the language research community, where people are understandably more interested in the power and facilities of their language than popularity.
Languages I know, want to use, and admire:
Erlang - One of the nicest languages I've come across. Scalable, safe concurrency, with pattern matching, and a whole gamult of features supporting fault-tolerant computing all the way down to low-level wire-protocol work.
Haskell - This is a language for thinking outside the box. Lazy, pure-functional, static-strong typing, advanced HM type inferencing, monads, arrows, the list goes on. If you have only ever done imperative procedual or object-oriented code (read java, C, C++, smalltalk, python, perl, etc) learn the exceptions to most of the rules you have osmosed.
Yes, I recognise the distinct lack of logic and constraint languages in this list so I'll just point at two I have been recommended but haven't found time to investigate sufficiently to add to the list above: Mercury, Mozart, and one that looks very promising Paul Reppy's Moby.