This extension provides goto, gotoIf and while loop functionality in Selenium IDE. Selenium IDE is a plugin for Firefox that automates the testing of web-based applications. There is an excellent flow-control extension at wiki.openqa.org/display/SEL/flowControl for the Selenium RC and TestRunner components, but it does not work with Selenium IDE (the Firefox plugin) directly. This makes it difficult to develop controlled test cases within Selenium IDE, and there are times when the frame-based TestRunner cannot be used (such as when the website under test employs a frame-buster script).
I've ported the existing control-flow extension to work in the Selenium IDE Firefox add-on. The image below shows a sample test case using goto, gotoIf and a while loop, all running successfully in Selenium IDE.
The file can be downloaded from Github and should be saved as "sideflow.js" to your hard drive. Then the Options settings in Selenium IDE should set the Selenium Core extensions to include the path of this file, similar to the image below.

github.com/73rhodes/sideflow
Update 26 February, 2008:
This mod of the FlowControl extension works only with the Selenium IDE Firefox add-on. If you want to use the TestRunner component or Selenium RC, please use the original FlowControl extension.
Update 19 February 2011
I've moved the extension over to github.
Update 7 July 2011
I've updated the example to use the newer syntax required in the latest versions of Selenium IDE.
Please note that if you are using Selenium RC, you should use the original flow-control plugin here: wiki.openqa.org/display/SEL/flowControl
Unfortunately I haven't had the time or the need to make this compatible with both Selenium RC and Selenium IDE. Please feel free to fork the code on github and make a pull request if you would like to help out.
Update 24 July 2012
I added a "push" command to the plugin, with a simple example on the Github page. This allows you to easily create a collection of items and implement "for each"-style functionality to iterate over it. Here's the original announcement.