GoogleCode is a hosting platform for open source projects that allows developers to host code, write documentation, track issues and bugs, tag versions, and distribute downloads. Mercurial is a distributed version control system that was created in 2005 as an alternative to Git and Subversion. It allows developers to work on code locally and remotely, manage versions, branches, and tags through commands like clone, pull, push, and commit changes. Mercurial offers advantages like being cross-platform, having a similar syntax to Subversion, not polluting the file namespace, and including built-in functionality for local and remote repositories as well as a web interface.