Feliway is a product that comes in a wall plug-in (like an air freshener) or as a spray.
It gives off "happy kitty" pheremones that can help soothe and calm cats. It doesn't have any detectable smell to people.
I swear by the stuff, but some others have said it doesn't really affect their cat; it probably depends on the cat. I use a plug-in at my house, and it makes a big difference with one of my cats. The other is pretty even-keel no matter what, but I can always tell when it's time to replace the plug-in (once a month) by the way Leo is acting. It takes a little for it to start working, and I think the product says to let it go for a month to get the full effect.
The downside to some people is that it's kind of expensive - I believe the refills for the plug-in are about $20-$25. I don't remember how much the original was.
You can buy it at most pet supply chains (i.e., Petsmart, PetCo, etc.).