Some commonly used tools include:
Low Orbit Ion Cannon (LOIC)
The LOIC is an open-source stress testing application. It allows for both TCP and UDP protocol layer attacks to be carried out using a user-friendly WYSIWYG interface. Due to the popularity of the original tool, derivatives have been created that allow attacks to be launched using a web browser.
High Orbit Ion Cannon (HOIC)
This attack tool was created to replace the LOIC by expanding its capabilities and adding customizations. Using the HTTP protocol, the HOIC is able to launch targeted attacks that are difficult to mitigate. The software is designed to have a minimum of 50 people working together in a coordinated attack effort.
Slowloris is an application designed to instigate a low and slow attack on a targeted server. It needs a relatively limited amount of resources in order to create a damaging effect.
R.U.D.Y. is another low and slow attack tool designed to allow the user to easily launch attacks using a simple point-and-click interface. By opening multiple HTTP POST requests and then keeping those connections open as long as possible, the attack aims to slowly overwhelm the targeted server.