It's summer time, and that means road trips, plane rides for business and pleasure, jaunts to the beach, and visits to the relatives. Experienced traveling parents realize that one of the most effective tools in their kid-quieting kits is a portable DVD player. Business travelers, for their part, appreciate the distraction afforded by being able to choose their own movies on the plane. Happily, there's a portable DVD player to fill every niche, from the least expensive units you aren't afraid to throw to the wolves in the backseat, to the sleek, silver traveling companions you won't be abashed to pull out of a briefcase in First Class.