More information on Meltdown (CVE Identifier: CVE-2017-5754): https://meltdownattack.com/
Here's the KB numbers and download links for the currently-supported versions of Windows.
This is supposed to have auto-installed on Windows 10 / Server 2016 already (if automatic updates is enabled), but it won't show up in Windows Updates for older Windows versions until next week (Patch Tuesday, January 9th), though.
Windows 10 (1709) and Server 2016 (1709): KB4056892
Windows 10 (1607) and Server 2016 (1607): KB4056890
Windows 8.1 and Server 2012 R2: KB4056898
Windows 7 and Server 2008 R2: KB4056897
KBs for other Windows 10 builds:
Windows 10 (1703): KB4056891
Windows 10 (1511): KB4056888
Windows 10 (1507): KB4056893
I haven't seen any mention of updates for Windows 8, 2012, Vista, 2003 R2, 2003, or XP, and I doubt there will be any official releases fixed for those. Those systems were already considered insecure for web usage, and now even more so.