Only a few knots are useful for <i>increasing</i> the tension on a rope.

<ul>
<li><a href="http://en.wikipedia.org/wiki/Taut-line_hitch">taut-line hitch</a> &mdash; often used for tents / canopies
    <ul>
    <li><a href="http://en.wikipedia.org/wiki/Adjustable_grip_hitch">adjustable grip hitch</a>
    </ul>
<li><a href="http://en.wikipedia.org/wiki/Trucker%27s_hitch">trucker's hitch</a>
<li><A href="http://www.shurdington.org/Scouts/GuylineHitch.htm">guy-line hitch</a>
</ul>

<h3>Ashley Book of Knots</h3>

<ul>
<li>generally, chapter 23, "Hitches to Stake and Post, Pile and Bollard",     knots 1773-1820
<li>1799-1801 &mdash; taut-line hitches
<li>2124-2126 &mdash; trucker's hitches
</ul>