FSinn
FSInn is the newest VATSIM Pilot Client, and is 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.
To download FSinn and the required FScopilot extension, click here.