I have a lot of software installed from sources other than the Apple Store. Is there a service to check which app have security updates ready to be downloaded? Opening each one on Fridays for a routine check sounds inefficient. For MacPorts, there is a script but I have stuff that comes from other sources too. Is this an unreasonable expectation to have? It seems technically feasible just to maintain a DB and query it with a list of package/version, package/version... getting back a list of update/version/url, update/version/url...
Edit: Better Option
I found AppFresh, which seems to do exactly what you're looking for, using a bunch of different sources, including Apple Update, Sparkle (which a lot of 3rd party apps use to push updates), Microsoft Updates and more. It also has the option of installing the updates right from the app.
MacUpdate offers RSS feeds of updates to all the software it tracks. You could use that, with something to filter for items matching software you have installed (Yahoo Pipes) might be useful for that) to give you a list of updates to any of your apps.
It's probably not a comprehensive solution depending on the obscurity of some of your apps, but it's probably the closest you'll get to a single source.
The other alternative might be creating a script that periodically checks provided URLs for all your software for any changes, then notifies you in some way.