Apple has released a set of security-related updates for Mac OS X. Security Update 2012-001 has been released for Mac OS X 10.6.8 (Snow Leopard), and Mac OS X 10.7.x (Lion) has been updated to version 10.7.3. These updates patch a large number of security vulnerabilities in multiple OS X components; more detail is available here. The 10.7.3 release of OS X Lion also has some additional capabilities.
The new versions can be obtained through the built-in Software Update mechanism, or from the Apple Support Downloads page.