http://code.edx.org/ The edX platform is implemented mostly in Python (with some Ruby and NodeJS as well) and the code is being made available under an AGPL license.
The main repository is edx-platform which includes both the LMS and the authoring tool, Studio. We have a number of other repositories outlined below, some of which may be useful even outside of the main platform.
If you are interested in writing components for our next-generation component architecture, take a look at XBlock. If you are interested in our machine-learning-based grading see EASE and discern. For our deployment and configuration tools, see configuration.