WebTester is a framework to test web applications. Designed to support several testing systems, it actually provides an implementation which uses Selenium RC.
You can test a any web application with it; but if you are developing Seaside applications you can take a component (any component), and unit test that component.
For me the cool part is that with this sort of tool it's easy to open the entreprise backdoor to Pharo. Pharo + WebTester can quickly integrate an existing (web) development based on other tools / languages.