AddonChecker by sbsmac
BIS created a set of “common” addons which were actually not common at all. It's therefore possible for players who do not have both games to run into problems when joining servers.
AddonChecker can tell you if a mission claims to be is “vanilla” ArmA 2 or Operation Arrowhead or to require both sets of content (Combined Operations or “CO”). More importantly it can detect CO missions by analyzing the classes used in the mission.sqm and and scripts run by the mission. It can then “fix up” those missions to prevent players accidentally joining the mission and then encountering a “missing addon” error message during the game.
More generally, AddonChecker can give you an idea before release or installation of what addons a mission might need - it's therefore useful for both server admins and mission-makers.