I have found a few products that were surprisingly effective or successful.
1) The Furminator undercoat brush. WOW!!! Super effective. If you have a pet that is a real shedder this is what you want. They have them for dogs and cats and for different lengths of fur and hair.
http://www.furminator.com/
2) Calming Collars. Again, WOW!!!! Not always effective, it seems to depend on the cat. But we had a serious problem cat who was so aggressive towards the other cats that he was going to have to go. But we [my x and I] put a calming collar on him and the effects were obvious in about a minute. In ten minutes he was a different cat. They have them for dogs and cats. But you want to be sure they are made from the synthesized hormone, and not herbal products.
They are particularly useful for short periods of time. If you move, or if there is some stress source like being alone, or just to help change the behavior of a problem cat, for any number of problems like peeing, destruction of property, attacking other cats... There is hope.
They have diffusers, as shown below, as well as collars [like a flea collar]. I recommend the collars.
3) Glucosamine with chondroitin
I have seen two animals brought back from near total incapacitation due to arthritis or joint injuries, back to a nearly normal life. My one cat is a prime example. She was limping very badly. But after a couple of months of glucosamine, the limp was almost completely gone.
4) Cat Grass - my cats LOVE it!!! They are indoor-only cats.
And now the two best toys I have ever discovered for cats.
5) Sheets of paper
6) Small boxes.
Yes, I buy a toy and they like the box better than the toy! LOL!