My main focus with respect to Sage has been working on Knoboo - Interactive Web Based Programming Notebook.
Knoboo is functionally similar to the Sage Notebook, but has some unique features such as it has a relational database backeded, has a decoupled frontend from the computational backend, as well as dynamic Google Docs like notebook management page and Mathematica like notebook 'Cells'.
Work is still in progress with Knoboo as it is still missing some of the great usability features of the Sage Notebook like @interact. Currently you can run Sage with Knoboo and Knoboo is planned for inclusion in Sage as soon as interaction with Sage is more robust.
My older Sage work: http://sage.math.washington.edu/home/agc/