Everyone knows that scientists experiment on rats, and guinea-pigs. That's why we have "lab rats" and why, if you're trying out something new, you're a "human guinea-pig".
But this is all out of date. Nowadays, mice are the most popular lab animals. Here's a graph showing the number of scientific papers published each year, mentioning each kind of critter (data gathered with this script):
Rats were on top until about 10 years ago, when mice overtook them. Why? No-one wants to study mice if they can help it: they are horrible to work with compared to rats, and rats are more similar to humans physiologically. This is why rats were more popular for a long time. (Contrary to popular belief, guinea pigs were never used all that much, and they've become even less popular with the rise of mice.)
Non-scientists tend to think of rats as just big mice. They're not: mice are less intelligent, harder to handle (they bite... a lot), and they smell bad. The fact that they're smaller makes surgery, and even simple stuff like taking blood samples, much harder. On the plus side, you can fit more of them in any given space, making them cheaper, but that's about it.
So why did mice suddenly claim the crown? One word - knockout. Mice are the only mammal in which it's easy to perform genetic knockout, i.e. eliminating the function of a single gene. It's extremely difficult in rats, because, for reasons no-one really understands, it is harder to get rat stem cells to grow in vitro.
Knockout mice were "invented" in 1989, and the inexorable rise in the number of mouse papers began a few years later. Recently, there have been reports that knockout rats may now be easy; whether this will lead to a rat renaissance remains to be seen.
Knockouts have revolutionized biology, because they make it easy to investigate what each gene does. Just knock it out, and see what's wrong with your mouse. This is why there are mouse models of so many genetic diseases, while rat and monkey models are only available for a few disorders.