Windows 8.1
From BetaArchive Wiki
Windows 8.1 is going to be an update to Windows 8. It's rumored that the update will be free. The codename is "Blue".
Builds
Windows 8.1 build 9220 - it is unclear whether this is a Windows 8 post-RTM build or a very early build of Blue/8.1.
Windows 8.1 build 9364 - first leaked Windows 8.1 build from Milestone 1
Windows 8.1 build 9374 - there was a partial leak of the PE and some files from System32.