One of my ThoughtWorks colleagues turned me on to this one. Most Mac OS X applications check for updates on startup (so many, in fact, that I'm surprised when I let something that doesn't do this get out of date). But each application has to do this on their own. That's where AppFresh fits. From the site:
AppFresh helps you to keep all applications, widgets, preference panes and application plugins installed on your Mac up to date. All from one place, easy to use and fully integrated into Mac OS X. AppFresh works by checking the excellent osx.iusethis.com for new versions and lets you download and install available updates easily.
Not only does it find the applications you need to update (which they mostly do themselves...mostly), AppFresh also handles the chore of downloading the DMG file, extracting it, mounting it, installing the application, and unmounting. No muss, no fuss. You just tell it to update an application and Voila...it's updated.