Visual Website Optimizer is an easy to use A/B testing tool featuring point-and-click test designer and WYSIWYG editor for creating variations. In addition to being dead-simple split testing software for marketers, it also dramatically shortens time to go live with your A/B tests because of the innovative "tag-less" integration (not to mention it reduces your dependency on IT and web development department for uploading code snippets again and again).
The Visual Website Optimizer plugin for Joomla integrates seemlessly into your Joomla website.
Update: Also works in Joomla 1.7
Update March 16 2012: The plugin now supports the new asynchronous code and now works in Joomla 1.5,1.6,1.7,1.8 and 2.5!