Get a free copy of Windows 8 RTM (90-day evaluation)
The final version of Windows 8 is now available from Microsoft as a 90-day evaluation. After 90 days, you'll need to do a clean installation of another operating system. This evaluation is intended for developers who are building apps for Windows 8 and IT professionals interested in trying Windows 8 Enterprise on behalf of their organization.
RTM = Release To Manufacturing (in other words, the final build that is going to sell on PCs)
You can get the Windows 8 RTM evaluation copy here.
I recently tried Windows 8 Release Preview (which is build 8400). The final RTM is build 9200.