There are a number of patch management solutions out there that check in with Microsoft to determine any applicable updates that may be available. Microsoft’s own WSUS (Windows Software Update Services) provides this free, but the same information is leveraged by a host of other third-party solutions that can provide more robust features. This is because Microsoft shares this information in a format designed for implementation by such products.
You can be notified of updates yourself via email by taking advantage of Microsoft’s security bulletin email notification service. You can register for this and other Microsoft newsletters at http://www.microsoft.com/technet/security/secnews/default.mspx.

Microsoft isn’t the only organization to send out security bulletins. Another good source for this sort of information is from the SANS Institute. You can subscribe to SANS newsletters at www.sans.org/newsletters. OVAL is yet another provider of security information that is used by many sites and management applications. In their own words, “OVAL is an international, information security, community standard to promote open and publicly available security content, and to standardize the transfer of this information across the entire spectrum of security tools and services”. Read more at http://oval.mitre.org.