something like Secunia PSI for Mac?

The Secunia PSI allows to scan, detect, check, and secure the applications installed on a computer. It also helps to secure applications, and to quickly and easily upgrade to new and secure versions of applications installed on a computer.

I'm not familiar with Secunia PSI, and this description doesn't help any. What exactly is it scanning, detecting and checking for? And what does "securing" applications mean? Is this anti-virus software? If so, you do not need virus software on your Mac.

Macs don't have spyware like Windows machines, and although there may be yet-undiscovered security flaws in the Mac OS that could be exploited by hackers, no software is going to find those for you. Known security holes are typically patched by security updates that you will automatically be notified of by Software Update.

As for third-party application updates, I believe that both [MacUpdate|http://www.macupdate.com] and [VersionTracker|http://www.versiontracker.com] have the capability to notify you when new versions of applications you have put on a "watch list" are available.

I'm not familiar with the details of this service for either, but the details should be on their sites.