PowerBlocks

PowerBlocks are the world’s best Dumbbells.
Why? PowerBlocks are perfectly balanced. They are the heaviest, most compact and easiest to use quick change dumbbell. PowerBlock is the only dumbbell system that can expand to meet your strength needs. Start with a 50 lb set, expand to 90 lbs, expand again to 130 lbs. Dumbbells are the most versatile and effective exercise tool ever developed for strength training. Now you can have rapid access to a whole rack of dumbbells at home in the space of just 1 pair, and at a fraction of the price of conventional dumbbells.
http://www.sportblock.com/