linum lets you display line numbers to the left of Emacs buffers.
linum-mode screenshot

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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