LOIC

Low Orbit Ion Cannon (LOIC) is an open source network stress testing and denial-of-service attack application, written in C#. LOIC was initially developed by Praetox Technologies, but was later released into the public domain, and now is hosted on several open source platforms.
The software has inspired the creation of an independent JavaScript version called JS LOIC, as well as LOIC-derived web version called Low Orbit Web Cannon. These enable a DoS from a web browser.

LOIC Website

DISCLAIMER: USE ON YOUR OWN RISK. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER OR CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES.