linum lets you display line numbers to the left of Emacs buffers.
As of Emacs 23.1, linum ships with GNU Emacs. You can
enable it with M-x linum-mode RET
You can also download either:
Copy the appropriate linum.el to your load-path and add
to your .emacs:
(require 'linum)
Then toggle display of line numbers with M-x linum-mode RET
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