Remove skin from chicken legs (use a paper towel to get a grip on the skin). Make a few slash marks in legs so marinade can soak in. Place legs in a glass dish or resealable plastic bag.
In a medium bowl, combine yogurt, lemon juice, ginger, garlic, curry powder, cumin, salt, cayenne and black pepper. Pour over chicken and turn to coat. Refrigerate overnight.
Heat gas grill to medium-high; for charcoal grill, bank coals to one side. Remove chicken from dish and scrape off as much marinade as possible. Coat with nonstick cooking spray and grill, bone-side down, 4 minutes. Flip over; reduce heat to medium. If cooking over charcoal, move legs to side without coals. Grill, covered 16 minutes more, until legs register 165° F.
Meanwhile, prepare Raita: Blend yogurt, cucumber, garlic, lemon juice and salt. Serve chicken over couscous, if desired, with raita alongside.
