1. In 2007 started Cython (http://cython.org/). We were using Pyrex (by Greg Ewing) for Sage (http://sagemath.org), and kept having to add new support and features. Pyrex was amazing, but Greg seemed to work on it once a year (?) by himself, with no revision control, and his view for the direction of Pyrex was somewhat limited. We needed something that could compile "99.9%" of Python, e.g., including list comprehensions, nested functions (closures), etc. I had added a bunch of things to Pyrex (in a fork), and a student of mine -- Robert Bradshaw (now at Google) -- had added a lot more. So I made up a good name ("Cython", for which the only Google search was a picture of a guy with a mohawk flipping the bird), made a nice website, and asked each of Robert or Stefan Behnel to be the lead developer. Both said, "NO", so I made them both co-lead developers, and amazingly that worked :-). A huge amount happened with Cython since then (many new devs, a book, etc.), with Cython now being very popular for writing fast compiled extension to Python for scientific computing applications. Happily I have done almost no further work on the Cython compiler, which is not the sort of work I like doing (that's why Sage uses Python, unlike almost every other math software package).

2. I wrote the Sage preparser, which adds a bunch of math-friendly syntax/language extensions to Python, in order to make it more suitable for interactive use for math. I realized this was needed while giving an early demo of Sage at PyCon in 2005. Otherwise, users of the competitor products to Sage would be much less likely to consider switching. This seems to have worked pretty well, and fortunately I think we haven't added anything to the preparser in years.