Lazy evaluation is what drew me into Haskell a few years ago, when someone showed me how you could use it to beautifully read a file as if it were in-memory, but done entirely as a stream. I thought that was amazing and I was sold.
I should have mentioned that in my initial comment; Lazy IO is beautiful and OK for a simple application, but it's smarter to use a streaming library like Pipes of Conduit for anything big.
