Ksplice is an extension of the Linux kernel which allows system administrators to apply security patches to a running kernel without having to reboot the operating system. Ksplice won the grand prize MIT Entrepreneurship Competition in 2009. Of course such an application is more needed at an enterprise level than as a regular desktop user.
You can download Ksplice from their download page. Also, if you are interested in the inner working of Ksplice, here is their technical paper [PDF