A lot of people seem to start off with a Jenny cage but I think they are terribly over priced and you can get something much bigger and better for the same amount of money. Its poorly made with small doors and uncoated bars which are all a no no in a good cage.
I actually use the Superpet Deluxe Multi Floor ferret cage which I love! But it is only suitable for adult male rats usually, as the bar spacing is rather wide. But it has endless possibilties for making the inside interesting and plenty of room for a small group of rats, my guys love it!