Scheme is a dialect of Lisp that I like a lot. Perhaps that's not strong enough: I don't think anyone needs to invent any more programming languages ever, since Scheme was invented (...and perfected with hygienic macros).
Why do I like Scheme, and think it's so fantastic?
- It's built from an almost-minimal set of primitive elements, assembled in predictable ways to provide interesting and emergent behaviours.
- It's got incredible hacker cred.
- It's extensible -- you can write new "macros" that work exactly like language primitives. The only other language I know that's more extensible is FORTH. And that's just too damn hard.
- It's stable -- in the last 30 years of its existence the changes to the language have been refinements, not revolutions.
- It's fun.
- It's useful for students and for professional developers.