Opera 12 is now in Beta, meaning it's ready for wider testing. If you're an Opera fan, there's no reason to skip this, Opera 12 Beta will install alongside your regular release and have a different profile. If you're not an Opera fan, there may be something in here you'll like. There are plenty of changes and new features in Opera, from the big and obvious to the subtle.
Native 64-bit builds for Windows and Mac
Also on the technical side, Opera 12 marks the debut of 64-bit builds for Windows and Mac OS. Linux/FreeBSD users had access to a 64-bit version of the browser, but one is now available for the more mainstream operating systems as well.
The native 64-bit version should run a bit faster in some cases and will enable the browser to use more memory than before, though few hit the 32-bit memory allocation limit.