Microsoft has seeded the Windows 8.1 build 9477 build to OEMs three weeks ago, which has subsequently leaked online.
Windows 8.1 will be officially launched October 18th and will come as a free downloadable update for users running Windows 8. It features a lot of enhancements under the hood, along with the re-inclusion of the Start button, changes to the Modern interface, which now includes different-sized tiles, and also comes with better social network integration.
Microsoft is trying to iron out the bugs before sending it out to OEMs, with sources citing August 23rd as the final day for internal testing and bug-fixes. As such, there were rumours indicating that 9477 will be the final build, but there have been screenshots of a 9484 build, so it looks like Microsoft is trying to get as many bugs as it can before hitting RTM. The latest leaked build, 9477, is in Chinese (9477 X64, zh-CN) and available online.