This is an article with a list of very different types of programming language all of which are worth learning: The seven programming ur-languages.
In addition to ALGOL languages (which every one needs to know), check out:
- Lisp: PLT Racket
- ML: Haskell
- Self: Self
- Prolog: Prolog
- Forth: gForth1
- APL: K (via ok)