Spaying/neutering isn't just for dogs and cats!
Here is a list of small pets that benefit from spaying/neutering:
Spaying/neutering rabbits is highly recommended. It allows male and female rabbits to live together without creating more bunnies, it prevents some kinds of cancer in rabbits, it also improves behavior. Many rabbits that aren't spayed or neutered can be aggressive, territorial (which means if you have a male rabbit it will spray urine everywhere and even if it is in a cage it can spray urine quite far!). Generally spayed/neutered rabbits are happier, healthier, and easier to live with.
Spaying/neutering helps prevent diseases such as certain types of cancer in rats, and allows males and females to live together without adding to the rat population.
Ferrets don't just benefit from spaying and neutering, but they need it. Males will be more aggressive, very smelly, and not as healthy if they are not neutered. Females, if they are not actively breeding, MUST be spayed, or they could die. Generally you don't have to worry about finding a vet for this yourself, because ferret breeders, pet stores, and rescues generally have all their ferrets spayed/neutered.
Other small pets:
I STRONGLY recommend NEVER spaying/ neutering any hamster, mouse, or gerbil. It is very costly, and most die in the process.
Guinea pigs can be spayed or neutered, but unless you want to put males and females together without them breeding, I wouldn't bother. And even then I would only neuter the male, because once the male is neutered, it isn't actually necessary to spay the female.
I am not sure about Chinchillas. Obviously if you want males and females together they need to be spayed, but I am not actually sure if it would benefit them besides that. And I don't know of any chinchilla rescues, so I don't know if rescues spay/neuter them or not. I guess I will have to do some chinchilla research!
Note: Fish cannot be spayed or neutered, and I am quite sure that reptiles and amphibians can't/shouldn't be either.