CodePlex is Microsoft’s open source project repository. It is built using Microsoft Team Foundation System and as such has until recently required the use of Team Explorer to access. However, due to the extreme popularity of Subversion(SVN), an open source tool for source control configuration, CodePlex has just announced support for TortoiseSVN (an SVN client). You can read the full details (including step-by-step instructions) in the official announcement, here, and access the root of the SVN tree via the web here.