A bit more than a week ago we had to experiment with Nessus as part of a class assignment. Nessus is a very complete vulnerability scanner that runs on top of Unix-based operating systems. In order to not get obsolete too quickly, the set of checks it runs can be updated based on a database maintained by the product's company, Tenable (much like what happens with an antivirus utility). It is important to note that this list is always seven days behind the up-to-date list unless you are a paid subscriber, which is very reasonable.
I liked how it worked and decided to try it at home to analyze my machines, so I went and downloaded the beta version for Mac OS X (I didn't want to fiddle with manual setup in other OSes...). After installation, it asked me for my activation code (sent by mail) and proceeded to download the most up-to-date vulnerability list (free version). At that point, it was possible to start the server part.
When launching the client I was presented with a neat, native Mac OS X interface. Analyzing the whole home network was trivial and the results were impressive. Despite that it raised some false positives (depending on the configured paranoia level), it told me several things that were sensible and listed pointers to external information (CVE entries, knowledge base articles, etc.) that was helpful to solve them.
If you are a network administrator, I bet this utility was already known to you but it came as new to me very recently and liked it.