One of the great features of the Mac App Store is the notification system that tells you when there is an update to one of you apps. However as we've seen, unless you buy all your apps through the Mac App Store, you won't receive notice of updates to apps purchased or download outside the store.
Luckily there is a very handy widget I've used for years (and that we've noted before) that automatically scours the net for updates to installed apps on your Mac. AppUpdate is the brainchild of Georg Kaindl and with the click of a button searches three software tracking sites: Apple's (soon to be defunct) Downloads page, MacUpdate, and Version Tracker (now owned by Cnet).
After running the widget, you'll see links all the updates to the apps on your Mac. You'll still have to click the links to go to the app's download pages, but running this widget is a lot less time-consuming than going to all of the software tracking sites and entering your apps manually, or even running the "Check for Updates" menu command that many apps offer. (If you prefer a non-widget update manager, both Cnet's TechTracker Free and the under-development Appfresh are worth checking out.)
The developer notes that he knows Version Tracker has been rolled into Cnet, and that he will be updating the widget to scan Cnet's Download pages in the future. As for the Apple Downloads pages, the scanning works for now. But even without these two sources, I've found MacUpdate always offers the most current listings of any app updates.
AppUpdate is a free download -- best of all it works on any Mac that is running Mac OS X 10.4 or later.