Aaron Swartz writes about the rewrite of reddit in Python. Apparently the Python framework used, web.py, was written by Aaron.
What I find most interesting is how quickly reddit was developed - and redeveloped. Only one weekend for the rewrite is pretty impressive. One reason for the fast implementation is because it's the second time they've written it and can learn from past design mistakes. They also get to reuse all the HTML design. It's a good case study though of how quickly a very popular site can be developed and maintained.
The reddit blog outlines why they rewrote reddit in Python, from the original Common Lisp.
If Lisp is so great, why did we stop using it? One of the biggest
issues was the lack of widely used and tested libraries. Sure, there
is a CL library for basically any task, but there is rarely more than
one, and often the libraries are not widely used or well
documented. Since we're building a site largely by standing on the
shoulders of others, this made things a little tougher. There just
aren't as many shoulders on which to stand.