The Secunia OSI identifies a few of the most common programs (complete list here: http://secunia.com/products/consumer/osi/programs_covered/), while the Secunia PSI can detect almost any program. In addition, the Secunia OSI is run using Java in a browser, while the Secunia PSI is a program the user downloads and installs. Both the Secunia OSI and Secunia PSI are completely free to use for typical home users.
I use it on demand only, and don’t have Java.