Selenium
Se Builder supports creating scripts in both Selenium 1 and Selenium 2, and converting between the two.
For Selenium 1, all standard step types can be added to scripts, exported, played back in-browser or using Selenium RC. See the Selenium documentation for details on these steps (actions).
Selenium 2 does not have a concept of step types as such, since it is a more free-form API. For reasons of compatibility and ease of editing, Se Builder defines step types closely patterned on those of Selenium 1. Not all Selenium 1 steps have a corresponding step yet, but work on this is in progress.
Within Se Builder, the Selenium 2 Step Support Table displays an overview of what steps there are, how they map onto Selenium 1, whether they can be exported to Java or Python, and whether they can be played back.
Scripts can be saved to a Builder-specific JSON format or exported as simple Java or Python code.
Most step types can be played back in-browser due to Se Builder incorporating the WebDriver plugin used by Selenium RC to play back tests.
RC playback is not yet supported, but is under development.