While there are a lot of testing tools out there to look for bugs, test a Website's scalability and perform unit tests, precious few seem to handle automating the testing of AJAX applications.  As a result, I wanted to start a list of tools that support automated testing of AJAX applications as well as debugging.  If you have other tools or applications you know of or something listed is out of date please add a comment to the blog and I'll get the information added/updated.

• ASP.NET AJAX Enhancements Library - The ASP.NET Ajax Enhancements Library is designed to provide an automated Unit Testing framework that integrates with VS.NET.  It also includes a client-side library that adds additional features to the Microsoft supported ASP.NET 2.0 Ajax Extensions Core library that do not currently exist in the Futures CTP.  The client-side library adds some nice short-cuts (haven't tried them yet...but they look interesting) such as $foreach and $break, as well as List and Dictionary classes.  It also includes a LINQ to JavaScript implementation.  Cost: Free
• Custom AJAX Testing - Article by James McCaffrey that discusses creating custom AJAX test automation scripts. Cost: Free
• jsMock - JSMock is a fully featured Mock Object library for JavaScript that provides the necessary tools to do effective interactive based testing. JSMock provides expectation recording and matching, and has the ability to return, throw, and stub on object method calls. The interface is simple and easy to use, which makes testing easy. Cost: Free
• jsUnit - JsUnit is a Unit Testing framework for client-side (in-browser) JavaScript. It is essentially a port of JUnit to JavaScript. Also included is a platform for automating the execution of tests on multiple browsers and mutiple machines running different OSs.  An article on using jsUnit can be found here.
• InCisif.net - InCisif.net is an automation tool designed to implement client-side functional testing of web applications under Internet Explorer 6.x or 7.x, using the C#, VB.NET or IronPython language.  You can read an article about using the tool for AJAX testing here.
• Selenium - A test tool for web applications. Selenium tests run directly in a browser, just as real users do. And they run in Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh. Cost: Free
• Sprajax - Although not a test automation tool, Sprajax allows AJAX applications to be scanned for potential security threats. Cost: Free
• Squish - froglogic Squish is a professional cross platform automated GUI and Web testing framework.  Squish allows to create tests for GUI and Web applications by recording or manually writing test scripts. Squish supports popular scripting languages for the test scripts and gives access to additional APIs. This way QA engineers can easily create powerful and robust tests to enhance and keep up the quality of the tested application. Cost: Unknown
• SWExplorerAutomation (SWEA) - This program creates an automation API for any Web application which uses HTML and DHTML and works with Microsoft Internet Explorer. The Web application becomes programmatically accessible from any .NET language. Cost: $99
• WaitiN - WatiN is developed in C# and aims to bring you an easy way to automate tests with Internet Explorer. An article discussing WaitN can be found here.  Cost: Free
• Web2test - Automated testing of web-based applications and portals, browser and platform independent, drag-and-drop, capture and replay.  Cost: 1.595,00 (euro price)
• WebAii - Caters specifically to automated testing of the "Web 2.0" generation of internet applications. WebAii is one the most advanced and rich .NET automation infrastructures available in today's market and is provided FREE of charge to the developer and quality assurance community. WebAii just recently released their Beta 2 feature set. With Beta 2 you will be able to execute your javascript unit tests directly from managed code and from Visual Studio Team System... for both firefox and IE. Cost: Free
• WindMill - Windmill is a web testing framework intended for complete automation of user interface testing, with strong test debugging capabilities.  Windmill is a web testing framework intended for complete automation of user interface testing, with strong test debugging capabilities. The first goal of Windmill to make test writing easier, portable and sustainable. The second goal of Windmill is to keep the project open and transparent, encouraging as much outside contribution as possible. Cost: Free

Several AJAX debugging tools are listed below:

• Aptana - The Aptana IDE is a free, open-source, cross-platform, JavaScript-focused development environment for building Ajax applications. It features code assist on JavaScript, HTML, and CSS languages, FTP/SFTP support and a JavaScript debugger to troubleshoot your code. Cost: Free
• 1st JavaScript Editor Pro - 1st JavaScript Editor is used for professionally editing JavaScript code and creating animations and other special effects for Web pages using DOM, DHTML, CSS, Ajax (Asynchronous JavaScript and XML) and JavaScript. Cost: $49.90
• FireBug - Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Cost: Free
• Fiddler - Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Cost: Free
• JavaScript HTML Debugger - SplineTech JavaScript HTML Debugger is an award-winning JavaScript Debugger that enables you to easily edit and debug JavaScript and VBScript inside HTML pages. Client-Side JavaScript, JScript and VBScript debugging languages are fully supported for simple and complex HTML and DHTML debugging scenarios. Cost: $89.00
• MyEclipse AJAX Tools - The MyEclipse AJAX / Web 2.0 toolkit features an enhanced JavaScript Editor and JavaScript Debugger (Professional Edition feature). Cost: $50 Annual subscription
• Script Debugger - The Microsoft Script Debugger is a debugging environment that extends any Microsoft ActiveX® Scripting host application—for example, Microsoft Internet Explorer (IE) or Microsoft Internet Information Server (IIS). Cost: Free
• Venkman - Venkman is the code name for Mozilla's JavaScript Debugger. Venkman aims to provide a powerful JavaScript debugging environment for Mozilla based browsers. Cost: Free
• Visual Studio .NET (and Web Developer Express) - Development IDE that provides integrated JavaScript debugging capabilities. Cost: Free and pay versions
• Web Developer Helper - Web Development Helper is an Internet Explorer plugin written by Microsoft's Nikhil Kothari that provides a set of useful tools to both Ajax/JavaScript developers as well as ASP.NET page and control developers. Cost: Free

This is a growing list so check back often for new entries.