CNET has created a fantastic new service called Tech Tracker that I wish I'd come up with. It tracks and keeps up to date all of the software on your computer! Mac or Windows! The only catch is that you have to create an account on their site and give them your email address. Hopefully it's worth it.
Here's the link: cnet.com/techtracker
One thing it got wrong, I just noticed, was that it thought I had Office 2007 installed. It must have found remnants of a trial installation that expired long ago. Anyway, otherwise it looks very useful.
UPDATE: It doesn't really keep anything up to date, it simply tells you what's out of date, leaving you to do the update yourself. At least they give you the link.
UPDATE: I have since (long ago) started using ninite.com, which makes updating very easy.