There's a lot of people giving good information on this thread.
But to the people saying that rats are bad pets, that they "breed like crazy and spread disease" Please for F*** sake stop spreading your ignorance.
Rats are among the smartest rodents and like I said the least likely to bite as they are far more domesticated than hamsters and gerbils especially if they have been hand raised.
They do breed like crazy but only if you are stupid enough to have a male and female together and TRUST me it's not hard to tell a male from a female as the males have extremely prominent man parts.
Domesticated rats aka Fancy rats also do not commonly carry diseases as long as you are careful about where you get them from. You do run a risk of getting a rat with an illness if you get one that came from a rat farm to be snake food. But even if you end up with a sick rat their illnesses do not spread to people and are usually curable with oral antibiotics that you can get from your vet same as any other house hold pet.
Their sinuses are extremely sensitive and they can be prone to respiratory infections but as long as you are using a non dusty bedding like yesterdays news, aspen or carefresh and you don't smoke around your rat then these issues can easily be avoided.
If you'd like a community that knows a TON about rats and can help you with more questions goosemoose.com is my favorite rat forum. There are rat breeders, rescuers and owners there that have a lot of info to share.