While reading a Haskell tutorial I came across the following code for defining the Fibonacci numbers:
fib = 1 : 1 : [ a + b | (a, b) <- zip fib (tail fib) ]
After reading it a few times and understanding how it works I couldn’t help but think how beautiful it is. I don’t mean that it’s aesthetically pleasing to me; the beautiful part is the meaning and simplicity. Lazy evaluation is sweet.
Haskell is the most challenging real language I have tried to wrap my head around. I haven’t done much with any functional languages yet but they are truly fascinating. I’m beginning to understand monads but I’m quite sure I don’t see the whole picture yet.
Erlang looks like it may be more suited to real world apps so I would like to learn that some time. The pragmatic guys have a book on Erlang in the works, and I love every book of theirs which I have read.
Going deeper down the functional rabbit-hole you’ll find things like this polyglot quine, which absolutely blows my mind. I used to be impressed by the JAPH sigs or some of the various obfuscated contest winners but that first one definitely cleans the rest up with a perfect 10 in geekiness.
 The following links have all been helpful while trying to wrap my head around monads.