Depends on the kid. xtramath.org is just straight facts practice, friendly, and to-the-point. I've found that smart kids who don't particularly enjoy fact fluency practice actually don't mind it as much because there's no BS to get through. You log on, practice (the guy makes funny faces and such, but there's no video game environment or anything), get your score, log off. Some kids like more "story," but for kids who just want to practice and be done, it's great. And free. And no ads. And easy.