Tough and durable, while still letting you move, the Protec Knee Pad Street Open Back is a great deal. A butterfly-style closure lets you take these pads on and off without removing your shoes.
Impact-absorbing foam covers the knees, and a shield of impact-resistant ABS plastic caps off the pads. Not only is the foam/ABS combo able to safely absorb most low-to-medium speed impacts, the caps let you slide out of any falls.