Cats may produce excess saliva at certain times which causes drooling. There are many reasons your cat may drool, but here are some of the more common ones:
"Happy drooling" - Similar to dogs, a small percentage of cats will actually drool in response to positive stimulation. So while you show your kitty some love, the purring and rolling over may be accompanied by a bit of drooling.
Mouth pain - An infection or foreign object caught in the mouth may cause your cat to drool. Be sure to examine your pet's mouth and seek veterinary care if necessary.
Heat stroke - If your cat is overheated, drooling can be a sign that he is experiencing heat stroke and needs immediate veterinary care.
Toxin ingestion - If they have a bad taste in their mouth or are about to be given poor-tasting medication, cats may begin to drool.
Other reasons cats may drool include: stress, mouth ulcers, periodontal disease.
If you notice unusual drooling behaviors from your cat (compared to what is normal for your cat) then it is wise to consult your veterinarian to be sure there are no underlying health concerns.
Khuly, Patty, DVM. "Why Does My Cat... Drool When She's Happy?" Vetstreet. N.p., 5 Jan. 2012. Web. 20 Aug. 2013.