Biggest python projects

Among many other Python-centered companies, beyond the ones already mentioned by Unknown, I'd mention big pharma firms such as Astra-Zeneca, film studios such as Lucasfilm, and research places such as NASA, Caltech, Lawrence Livermore NRL.

Among the sponsors of Pycon Italia Tre (next week in Firenze, IT -- see www.pycon.it) are Qt/Trolltech (a wholly owned subsidiary of Nokia), Google of course, Statpro, ActiveState, Wingware -- besides, of course, several Italian companies.

Among the sponsors of Pycon US in Chicago in March were (of course) Google, as well as Sun Microsystems, Microsoft, Slide.com, Walt Disney Animation Studios, Oracle, Canonical, VMWare -- these are all companies who thought it worthwhile to spend money in order to have visibility to experienced Pythonistas, so presumably ones making significant large-scale use of Python (and in most cases trying to hire experienced Python developers in particular).

Youtube is probably the biggest user after Google (and subsequently bought by them).

Reddit, a digg-like website, is written in Python.

Eve, an MMO with a good chunk written in Python is pretty impressive as well.

https://en.wikipedia.org/wiki/Python_(programming_language)#Uses

https://en.wikipedia.org/wiki/List_of_Python_software