Simple is a portscanner. It used to be very simple at the beginning but it soon got a bit complex. Initially it was a program which scanned a class C network for computers with windows shares. Before some years there were lots of computers with unprotected shares. Now you will hardly find some. But Simple still works. The big hit for it was the Service Pack 2 for Windows XP. For some stupid reason (to stop your computer from doing DoS attacks or something) Microsoft decided to put a hardcoded (= not something that can be changed like a normal parameter) limit of 10 on the concurrently half-open connections. Fortunately some guys made a great toll to modify this limit (some antiviruses detect this tool as a virus but you can safely ignore them).
You can download Simple by clicking here.
Here is a screenshot: