Functional languages

I want to learn functional programming. What language should I use?

Common Lisp
bigger community and usage, people like Alan Kay or Dijisktra said it opens your mind to higher levels, macros, Stallman recommends it

Haskell
Strong typing, pure functional, less weird syntax

Well, learning functional programming can get a bit involved.

The first two chapters of SICP deal with the basics of functional programming, employing Scheme. It's very bare-bones, using only function definitions, variable binding and anonymous functions. You can use those constructs in almost any modern language (yes, that means that Java isn't modern), though poor language design or excessively imperative standard libraries can hinder your use of them.

In Haskell you get a pattern matching, list comprehensions, a nice type system, built-in and fancy ADTs, though at least the first two you can get as libraries in any Lisp dialect. I can't comment much more, I'm still learning from the official tutorial and Learn you a Haskell.

I'd go with Scheme first, and Haskell second, even if it's just to get a taste of a state-of-the-art language.