IPython looks amazing. I haven't personally used it yet but it appears to bring most of the Mathematica-notebook goodness over to the python community. The one thing it is lacking, which is admittedly pretty hard to implement, is Mathematica style equation entry. I can input an equation in Mathematica much faster than I can in Matlab or python.

Markdown is a real pleasure to use. One of my favorite features is that it is easy on the eyes. I was never in love with the Latex syntax.