The developers behind reddit are releasing their social bookmarking software into open source.

Reddit means with 'open-source' 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.

There are 5 developers behind the reddit code.

All reddit code is licensed under the Common Public Attribution License, which is basically the Mozilla license with a handful of changes. Specifically, the CPAL stipulates that when running reddit's code publicly, any changes to the code must be made available publicly and the site must make clear that it is running reddit code.
Via this reddit blog post.