Boon means something beneficial to a specific person, entity, or cause. "Getting called out of school on the day of the test was a boon for Sam, as he hadn't remembered to study."
Boon derives from the Old Norse bón, a request for a favor. Think of a boon as a favor that no one has necessarily asked for, something extra. "We'd just spent our last dollar on a cup of coffee so it was a real boon to find a ten dollar bill lying on the sidewalk." Boon can also be an adjective for someone friendly and welcoming, as in "a boon companion."