Some projects are able to extend SimpleTest's functionnality :

Limb Unit

limb_unit is an advanced SimpleTest based tests runner utility for PHP5. In a nutshell, limb_unit finds and executes tests within the file system. It's similar to phpunit utility from PHPUnit, yet more powerful.

SimpleTestXUL

A XUL interface to SimpleTest framework.

Stagehand_TestRunner

Automated test runners for PHPUnit and SimpleTest.

Suite Tester

Suite Tester is an AJAX-driven add-on for the PHP SimpleTest unit-testing tool. The main benefits are conveniently testing an entire application or a single unit in one place, and quickly viewing a suite's results one unit at a time.

Others are using SimpleTest to build stuff :

CakePHP

CakePHP is a rapid development framework for PHP which uses commonly known design patterns like ActiveRecord, Association Data Mapping, Front Controller and MVC.

Web Application Component Toolkit

The Web Application Component Toolkit is a framework for creating web applications. WACT facilitates a modular approach where individual, independent or reusable components may be integrated into a larger web application. WACT assists in implementing the Model View Controller pattern and the related Domain Model, Template View, Front Controller and Application Controller patterns.

Limb3

Limb is an OpenSource(LGPL) PHP framework aimed for rapid web application development. It is a committed proponents of the beautiful and easy-to-maintain code that simply works. That is why the Limb code is developed and constantly refactored in a test driven manner using the best agile development practices.

Drupal | SimpleTest

A framework for running automated unit tests in Drupal. This module comes with a set of core tests. These can be helpful when making patches for the drupal core or when building your own set of modules.

WideImage

WideImage is an object-oriented PHP image library, written in/for PHP5. Uses GD2 and is unit-tested with Simpletest. Promotes ease of use and extensibility.

Aperiplus

Aperiplus is a general purpose, unit-tested, OOP library for PHP5. It contains extensions for SimpleTest.

Flux CMS

Flux CMS is a XML/XSLT CMS based on PHP 5 and Popoon. It's easy to use for the enduser and has WYSIWYG editing capabilites, but it's also very extensible and powerful for implementors to suit your needs.

Moodle

Moodle is a course management system (CMS) - a free, Open Source software package designed using sound pedagogical principles, to help educators create effective online learning communities.