One of the weird historical accidents of programming languages is that so many of them use $ for important things. The reason is just that out of the available punctuation, nearly all of it has a mathmatical or other predefined use that makes sense to retain in a programming language context, while $ (and also @ and #) do not. Still, $ annoys me, it's so asymetric that we use it all over our code, and never a £ or ฿ to be seen.

The one language that manages to use $ nicely, IMHO, is Haskell. Recently I noticed that it has an actual visual mnemonic in its use of $. And it's used for something I've not seen in other languages.

The visual mnemonic of $ is that it looks like an opening parenthesis, with the related closing parenthesis on a line below it.

(something (that
    (lisp folks
        (are (very (familiar with)))
    )
))

And this is also the problem that $ solves:

something $ that $
    haskell folks $
        are $ very $ familiar with

This is a trivial feature.. but oh so useful. The implementation in Haskell of $ is simply:

f $ x = f x
infixr 0 $

Just function application, but at a different precedence than usual.

I am now very addicted to my $. Out of 15 thousand lines of code, only 87 contain )), while 10% use $.