This isn't really for exercising, but it is a necessity for some people, because a conventional scale just won't do. Fortunately, this professional-grade scale weighs people up to 550 pounds.
Burst-Resistant Stability Ball
Unless you tip the scale at over 2,000 pounds, here's one exercise ball you don't have to worry about popping.
Extra Sturdy BIG Bicycle
Need a bike that can support more than 300 pounds? You're in luck, thanks to the recumbent bicycles and tricycles offered by Lightfoot Cycles. They feature over-sized wheels and sets made specifically for overweight cyclists.
Bike seats are notoriously uncomfortable, and are even more so for people who carry a bit of their own "extra padding." For them, there are twin-pad, high-capacity seats.
Designing an exercise program and activities for someone who is extremely overweight can be a challenge for a personal trainer. Here in Toronto my focus when helping such people to lose weight is to get them outside and doing fun activities that they can not only do, but will enjoy doing on their own even when I am not there to encourage and motivate them.
Having the necessary equipment so you can exercise like a normal person and not feel like you are disabled makes a big difference in your ability to stay motivated and keep exercising.