imenu

Purpose of this package:
  To present a framework for mode-specific buffer indexes.
  A buffer index is an alist of names and buffer positions.
  For instance all functions in a C-file and their positions.

  It is documented in the Emacs Lisp manual.

How it works:

  A mode-specific function is called to generate the index.  It is
  then presented to the user, who can choose from this index.