Linux: If you're running a server, always-on media box or other system that's powered by Ubuntu, you probably don't love having to reboot just to install security updates. Ksplice, a semi-free service, eliminates the need for downtime.
Ksplice operates in way strikingly similar to Ubuntu's built-in Update Manager, checking for the latest vulnerabilities and fixes posted by Linux system administrator and builders around the globe and pinging you when updates are available. The difference is that Ksplice makes the fixes without requiring you to reboot at all.
It's a "semi-free" service because, unless you're specfically running the latest edition of Ubuntu, the 9.04 release known as Jaunty Jackalope, you're either out of luck or contacting Ksplice to cater their services to your system as a paid service. Regardless, for home server enthusiasts looking to maximise uptime, or server administrators willing to try out a clever updater, Ksplice is worth checking out.
Ksplice is free for users of Ubuntu 9.04 Linux only, but offered for any Linux-based system (and, potentially, other systems in the future).