"Secunia also markets a commercial version of its security inspector to corporates, called Online Software Inspector, and an Enterprise version of the software, Network Software Inspector."
Slight mistake there.... OSI is in fact the original free version of the tool and is basically a cut down version of PSI that runs in a browser for those who aren't allowed to install software. NSI is the only commercial version.
@ Yeah, But: No it doesn't just grab a list of software from the registry, it actually scans the whole drive and tells you everywhere that you have out of date software installed. (If you set it up correctly.) Including all those old versions of Java and Flash that their updates never remove, old versions that other apps have installed in unusual places, etc. Having used PSI since the first beta I'm always impressed that it detects all the open source and unusual apps that I run. Try it, I bet it discovers all sorts of unpatched vulnerabilities on your system you had no idea about.