We've always strived to be as open and transparent with our users as possible, and making reddit open source is part of that. When we say "open-source" we mean specifically that the code behind reddit is available to the public for download, and we're inviting the public to submit code to help improve the site.
reddit is unique in the social news scene in that we have a huge community of developers. It seems only natural that we give you all in that community a chance to contribute back to reddit and make it a better place for everyone. We know reddit's success has less to do with our technology than it does with you, our community, and we want to let our community improve our technology.
For Ubuntu 12.04 (Precise Pangolin) we have an automated install script that sets up a suitable development environment.
When you've finished working on your patch, send us a pull request and we'll look at merging up your changes. For simpler changes (template tweaks, CSS fixes, etc.) you're welcome to send unsolicited pull requests. For larger, more core, changes make sure to check with us first so you don't waste any time if we don't feel it's the right direction to go down.
When you've submitted a pull request, we may respond with bizarre emoticons. Read up on The Code Salon for info on our code review process.
Note: these channels are for discussing development of reddit's code, if you are looking for a place to discuss open source in general, see #reddit on freenode.