There is a few contraceptives available for cats, and like stated, Ovarid is one. However, many are associated with terrible side effects. They must be used with caution, and after a lenghty discussion with your vet to see if they really are the best option. They are nothing like the the human varieties, and shouldnt be routinely used.
NOAH Compendium of Animal Medicines: Ovarid - Introduction
This is the info on Ovarid (it carries the least risk for side effects), i cant remember the others right now.