PowerBlock Dumbbell system
I just ran across the PowerBlock. They look solid, a quick pin switch and you have a new weight Dumbbell.
If anything, these are SmartBells. PowerBlocks seem to be built well, The Pins eliminate any possible hardware failure, and the space saving is amazing (it would take a 20 foot wall to hold the same number of dumbbells).
They have a few flavors:
