For general numbering of lines, there are two packages for use with LaTeX, lineno (which permits labels attached to individual lines of typeset output) and numline.
Both of these packages play fast and loose with the LaTeX output routine, which can cause problems: the user should beware...
If the requirement is for numbering verbatim text, moreverb or fancyvrb (see including files verbatim) may be used.
One common use of line numbers is in critical editions of texts, and for this the edmac package offers comprehensive support.