Also have to chime in with the rat-support. Rats are fantastically, wonderfully, amazing little beasties–far and away the best of the pocket-pets. They are bright, clean, charming, friendly, remarkably tolerant, and way too smart for their own good :).
They need less cage cleaning than cavies, don’t smell as bad as hamsters (MUCH less evil, too. Hamsters are of the devil, and not in a good way), and are far more cuddly and friendly than chinchillas. Their only downside is their short life expectancy–two to three years, typically 
--however, this can be mitigated by buying ratties from a hobby breeder who breeds for health and longevity, keeping them on a paper-based bedding, and feeding a varied diet of healthy foods. I bred rats for about five years as pets, and was attaining life-spans of upwards of four and a half years in my litters. Also I have to vehemently disagree that they’re okay on their own–one of the reasons they make such good pets is that they are extremely social. A solitary rat will get sick much more easily, tend to be shy, neurotic, and generally extremely unhappy. It also seems to drastically shorten their life-span.
Hamsters are very short tempered–unlike the majority of animals we keep as pets, they are solitary and thus do not need or desire companionship the way most other animals with which we share living space do. Therefore, you’re likely to end up with a critter that would rather you just leave it the hell alone, thank you very much. Plus, they’re desert animals and conserve water–their urine is extremely concentrated and thus very, very strong smelling. Life expectancy, 1.5 years for Russian Dwarves, 2-3 for any of the Syrian varieties.
I don’t know much about gerbils, as they are illegal to keep as pets here, but I have interacted with them in a research environment. They seem a lot like hamsters, but more jumpy. They do backflips, though, that’s pretty cute.
Rabbits are a lot of fun, but just as much work as a cat, if not more so. If kept as a house-bunny, and not stuck in a hutch all day, they make fantastically fun and intelligent pets. They litter-train very easily, much like cats, and will learn to play games like hide and go seek, and peek-a-boo. They’re much brighter than most people give them credit for, I think. Watch for chewing on electrical cords and digging carpets, but there are fairly easy ways to solve or circumvent these problems. Bigger breeds are typically much friendlier and more laid back than mini or dwarf breeds. Netherland Dwarves are rather testy. Average life expectancy, 7-12 years.
Guinea pigs (cavies) are very vocal, talkative little beasties. They pee a lot, and eat a lot. Very friendly with a lot of handling as babies, otherwise can be skittish. Mine learned to recognize the sound of the refrigerator door opening and would start making “eep!” noises immediately 
Average life expectancy, 6-8, up to 10+ years.
Chins are the softest animals on the face of the planet–literally. They have denser fur than any other animal that exists. Too bad they don’t like to be held, heh. They’re very high energy, very intelligent, and very, very nocturnal. Reccomend not keeping the cage in your room if you’re a light sleeper. They’ll sit on your open palm, or climb on you, but prefer not to be held or confined by hands. They’re not unfriendly, but tend to like to interact with you on their terms. Feed a very strict diet of chin pellets and timothy hay and a little grain with the occasional raisin as a treat. They’re also way more expensive than the others, if that’s a factor, and need a much larger, much more elaborate cage. Life expectancy, 12-15 years and sometimes up to 22(!).
Oi, that’s long. If you have any other questions, feel free to ask.
Peace,
~Mixie (small animal specialist and rat fan)
