While no direct open-source equivalent exists for S4U Stair, FreeCAD and Blender have robust stair-generating capabilities and are completely free. You can model stairs there and import to SketchUp.
Intrigued, Alex decided to give the S4U Stair Plugin a try. The first thing that struck him was the straightforward installation process and the intuitive interface. Even with a steep learning curve for some of the more advanced features, the plugin's documentation and support forums were incredibly helpful.