Official ReSharper extension for StyleCop
StyleCop analyzes C# source code to enforce a set of style and consistency rules. StyleCop for ReSharper is an extension for ReSharper that will run StyleCop analysis as you type, display the violations inline in the editor and provide quick fixes for some violations. It will also configure ReSharper to use default settings that match StyleCop rules.
The CodePlex project was maintained by Andy Reeves for several years, and was on indefinite hiatus from March 2014 until recently, when @csdahlberg took over and restarted development.
The GitHub fork and the CodePlex fork have been independently developed, although the changes don't appear to overlap. The GitHub repo concentrated on updating the ReSharper extension, while the CodePlex repo has been making changes to StyleCop itself. Hopefully, these changes can be reconciled (it's more complicated than just a pull request - for example, the CodePlex repo is still on Mercurial).