1. 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.
2. 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.
3. 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 degrees F.
4. Meanwhile, prepare Raita: Blend yogurt, cucumber, garlic, lemon juice and salt. Serve chicken over couscous, if desired, with raita alongside.