I really like the new Filehippo Update Checker. It scans your installed applications and checks to see if you have the latest versions installed.
This is important because it's not just your operating system that can contain security vulnerabilities that need patching - applications need patching too, and many of your standard installed applications such as Realplayer and Adobe Reader are regularly updated by the vendors to plug security holes.
I also love the fact that this update checker is only 100k and doesn't need installing. You can just run the exe and off it goes - no installation files that adds the app to your Startup folder or makes changes to the registry of system files.