Microsoft has reached the release candidate stage for their upcoming Windows 7 operating system -- the successor to Windows Vista.
You can download the full version in either 32 bit or 64 bit at
http://msdn.microsoft.com/en-us/evalcenter/dd353271.aspx.
A release candidate means exactly that, it's a version that is being considered as the version that will be RTM (released to manufacturing), thus the version you'd buy in the box or on a new PC. Therefore this is much more stable and faster than the previous BETA version released in February (which I'm working on right now). It's normal, however, for there to be more than one "RC" versions over the course of final tweaking and testing, but those are likely to be released privately to a small number of testers.
This version is fully functional until March 2010, after which time you'll need to buy a regular license. The RC version formally expires July 2010, but between March and July you can use this Windows 7, but your machine will shutdown every two hours.
Look for a full preview of Windows 7 in the near future.