Workspace is a project mainly written in JavaScript, it's free.
simple JavaScript workspace
A Workspace is a simple editor for entering and executing Javascript on a content window. Enter some text, select what you want to run and pick Execute from the context menu to evaluate it against the currently-active tab in a browser window.
For an installable version, please visit the Add-ons page
From inside the workspace project directory, run: zip -r ../workspace.xpi -x ".git/"
All Copyright dedicated to the Public Domain.
0.5.3 [2011-02-16] Removed close button on PropertyPanel, incorporated erikvold-issue-20.
0.5.1 [2011-02-03] changed WS to WORKSPACE per AMO review request.
0.5 [2011-01-26] chrome sandbox, edit menus operational.
0.4.4 [2011-01-23] added shortcut key to error console.
0.4.3 [2011-01-23] replaced statusbar text with chrome/content status.
0.4.2 - [2011-01-23] bug fix for issue 11.
0.4.1 - [2011-01-23] Added Tools menu and Error Console access. (erikvold-issue-9)
0.4 - [2011-01-13] Added File functions.
0.3.1 - [2011-01-12] Revved maxVersion in install.rdf
0.3 - [2011-01-09] Chrome and Content context menu options working.
0.2 - [2011-01-08] Menus roughed in, execution of whole window or selection. Some cleanup.
0.1 - [2011-01-05] initial prototype, basic execution
Lots. This is a prototype.