AppFresh
If you’ve ever used a package manager on Linux (apt-get, dpkg, pacman, yum, etc.), then you know how easy it can be to update all of your installed software at once. AppFresh is an application for OS X than lets you keep all applications, widgets, preference panes, and application plugins up to date.
I’ve been using AppFresh for over two years now, and I love updating all of my apps from one location, versus launching each app individually and checking for updates.