This is a utility that allows you to check the version of your non-Digidesign plug-ins to determine whether or not they need to be updated for use with the currently installed version of Pro Tools.
To use the Digidesign Plug-In Validator:

Launch the "Digidesign Plug-In Validator" application located in your Pro Tools Utilities folder. If any of your non-Digidesign plug-ins have not been updated, they will be listed in the display window. Please visit the Web site for the manufacturer of any affected plug-ins and download the update (if available), or ask the manufacturer when one will be available.

This utility was originally introduced with Pro Tools 6.2.2 due to a PACE copy protection bug. Some users experience instability and/or crashes when launching Pro Tools, opening a session, or dismissing a trial-period dialog if any Pace-wrapped third party plug-ins are installed. The fix for this bug is in the latest PACE drivers included and installed automatically within the Pro Tools software installer. Updates to all plug-ins are also necessary to prevent instability and/or crashes. All plug-ins supplied with the Pro Tools 6.2.3 installer have been fixed. Even if you do not have updates for all your plug-ins, instability would only occur immediately after adding or removing a plug-in from your Pro Tools Plug-Ins folder. After the first crash and subsequent reboot, you should not see any further problems, unless you then add or remove plug-ins from the Plug-Ins folder.