We have recently moved our source code from Subversion to GitHub. GitHub is a popular and hosted source code control system based on Git, an advanced successor to Subversion.
We've made this transition because it simplifies our software release process, as we can now more easily work on two versions of our code simultaneously - our current stable series (e.g. v2.4.x) as well as upcoming releases that contain vastly different code (e.g. v3.) GitHub makes it much easier for developers outside the company to contribute code to us. This is important because, getting our community more involved with core development is a key part of our plans for SilverStripe 3.0.
Code on Subversion is now frozen, and all of the SilverStripe core and modules can now be found on http://github.com/silverstripe, including SilverStripe CMS v3.0 code and user interface design work. Our more experimental work is at http://github.com/silverstripe-labs/.
If you are used to downloading SilverStripe code from our own website, nothing will have changed for you, but if you have previously accessed SilverStripe from Subversion, you should access it from GitHub now; instructions are here: http://doc.silverstripe.org/sapphire/en/installation/from-source.