TechTracker

CNET has a program called TechTracker which scans your computer periodically and informs you of application updates. The only downside is that you need a cnet login (free) to access a webpage the application generates with the upgrade report.

I'm running it at work (on xp32, it's fine). At home it's not loading the report page corretly (Vista64), though according to CNET it should.

You can try it out here.