It's just as the title says.
Thanks.
How does this look:

Whilst I generally favour TikZ for anything vaguely graphical, this was achieved with a simple \rule:
\documentclass{article}
\thispagestyle{empty}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\noindent\rule[0.5ex]{\linewidth}{1pt}
\lipsum[2]
\end{document}
You can thicken the line by changing the 1pt to something else (or make it thinner, I guess). Perhaps style purists would want to change the before and after spacing a little.
(Added in edit: in the comments, Style Purist Will recommends adding a little height, and as it's so easy to do - I'd forgotten about the optional argument, myself - I thought it worth adding in the demonstration. He also used \linewidth rather than \textwidth - my original choice - which is a little more robust as it will vary correctly in lists and other environments where the linewidth can get altered.)
\rule[0.5ex]{\linewidth}{1pt}. I guess that makes me a style purist, whatever that is ;)
– Will Robertson
Oct 13 '10 at 12:54
\noindnent\hrulefill or even just \hrule ? Just for the convenience of changing the width or length more easily?
– frabjous
Oct 13 '10 at 18:30
\noindnent, what does that do?)
– Andrew Stacey
Oct 13 '10 at 21:39
\noindent with \rule seems strange. I wonder why it wasn't written to just work in either h mode or v mode. Looking at the definition of \@rule, it'd be a trivial modification: remove \leavevmode\hbox and replace \vrule with \ifvmode\hrule\else\vrule\fi
– TH.
Oct 14 '10 at 09:10
In ConTeXt, \blackrule gives a horizontal line. By default, the width of the line 1em wide and 1ex tall. The width, height, and depth are changed using the respective options. For example:
\blackrule[width=\hsize, height=1pt, depth=0.5ex]
Despite its name, \blackrule also draws colored rules.
\blackrule[color=red]
\setupblackrules specifies the options for all \blackrules.
As a bonus, \blackrules (notice the plural) draws multiple horizontal rules. For example:
\blackrules[n=6, width=\hsize, distance=0.5ex]
draws 6 rules separated by a distance of 0.5ex.
I typically use \hrulefill or \noindent\hrulefill to achieve a horizontal line.