Which Apps on my Mac need security updates?
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.



Original Answer

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.