Less Wrong is a fork of the open Reddit codebase, written in Python. Less Wrong's code is online at Github (look in r2/r2 for the meat), the issues tracker is at code.google.com. See Contributing to Less Wrong for a gentle introduction to getting started.
According to Reddit's blog, we are the coolest use of reddit source code they've seen! But we've still got a long way to go before we're as cool as we want to be.
If anyone out there is fluent in Python and willing to donate a noticeable amount of time to a good cause, an extra hand or two might help us implement many Less Wrong features a lot sooner.
The Reddit codebase does not have unit tests or a whole lot of documentation. Contributors need to be able to wade through Reddit's code to grok it, and write unit tests for what they do (or better yet, write unit tests for existing code).
Items on the issues tracker marked "Contributions-Welcome" are those that look relatively easy to contribute. Items marked "Contributions-LeaveItToUs" are those that look big and complicated, or that Tricycle (the main developers) have strong opinions about how to design and implement. You could hack the big ones, but the developers might need to spend time talking to you - so please don't step up unless you're fluent in Python, have the necessary time, and are serious about it.
An example of a Welcome contribution would be having the registration page explain what is a valid username, or making sure that any HTML generated automatically in comments is also legal to enter directly (like <a href></a>).
An example of a LeaveItToUs contribution would be the future Tag and Sequence system - a next/prev tracking system for tags (so that you can navigate through via "next in self_deception" / "prev in self_deception" arrows); a system that lets authors create sequences which behave like tags but are owned by that author; and RSS feeds for tags and sequences that feed a specified number of posts per day to new users trying to catch up. An experienced volunteer Python dev with enough block hours free to run out and do this would be very welcome.
Hopefully this gives you an idea of what kind of work needs doing. See the issues tracker for more.