FSInn is the newer Pilot Client, and are available in two versions, one for FS9/FS2004, and a beta-version for FSX.

In many respects, this Pliot Client differs quite a lot from SquawkBox, by adding more windows and visible applications to the pilot, while still managing its primary task, namely being the connector between your flightsimulator and the VATSIM Network.

However, as the FSInn webpage http://www.mcdu.com/ has been down for quite some time, the best suggestion is to look at the VATSIM forum, under FSInn, to find both updated and relevant information.