Today is the last day that all three previously released free Windows 8 preview versions will continue to work without them restarting the PC every hour until a full Windows 8 license key is purchased From Neowin:

If you are one of the few remaining people who still uses a free preview versions of Windows 8, it's now time to say goodbye. As we have previously reported, today is the designated date for all three preview versions of Microsoft's latest OS to expire.

Technically, those early versions of Windows 8 will still work. However, as revealed on this Microsoft support page, users of the Windows 8 Developer Preview, Consumer Preview and Release Preview will soon see this message on the Activate Windows menu selection in Control Panel

Today is the day to say "Goodbye" to all three Windows 8 preview versions