Oracle has released a new version 6, update 29, for its Java run-time environment, for Windows, Solaris, and Linux. (Apple packages and releases its own version of Java for Mac OS X.) This version contains fixes for numerous bugs, including 20 security vulnerabilities, some of them serious. More information is available in the Update Release Notes, and the Critical Patch Update Advisory. (Incidentally, the last released version was Java 6 Update 27; there was no released Update 28.)
If you have Java installed on your system, I recommend that you install the new version as quickly as you conveniently can. Windows users can use the built-in automatic update mechanism; alternatively, the new version can be downloaded here.