Similarly, !! refers to the last command, so you can write "sudo !!" to execute the last command as root. Helpful for apt-get, among other things.

!$ refers to the arg to the last command, so you can write:

  $ ls -l /foo/bar
    # .oO(aha, it's a stale link i don't want)
  $ rm !$
  rm /foo/bar
  $

I have that one aliased as sl2ls ...