In Emergency 3, you control a city's first responders -- EMTs, fire and rescue services, police, and so on--responding to emergencies. In twenty missions, you have to deal with a wide variety of them, from raging fires to explosions, derailed trains carrying dangerous chemicals, etc. There's also a 'sandbox' mode with randomly-generated emergencies, so once you've completed the missions, you can continue to play indefinitely, if you like. It's a real-time strategy game, in a sense, but your objective is saving lives, rather than conquering enemies.
It's a high-level development effort, too; fully 3D with nicely rendered graphics of the city you're protecting, realistic physics, and a wide variety of unit types to command. We like the game for its subject material, and the depth in which it treats it. (It's a big download though--over 600 MB. You'll need a broadband connection, and it'll take a while.)
The game is not, however, without flaw. For one thing, because of the diverse capabilities and characteristics of the units represented--ambulances, police helicopters, and many different fire vehicles, to start with--the interface is fairly complicated. That's perhaps necessary, to provide so detailed a sim, but this is far from a "one button" game.
For another thing, many of the missions are quite unforgiving--if you don't respond quickly and appropriately, you can easily lose by, e.g., failing to realize that you need to divert traffic on a highway where an emergency is occurring. In other words, if you miss a pesky detail, or don't plan your reponse adequately, you can quickly get into a no-win situation. The problem is compounded by the fact that while there's a tutorial, it's pretty short and doesn't teach all the capabilities of the units you command, so there's sometimes a degree of frustration in figuring out where you went wrong, and what you need to do to fix the problem. The upshot is that you can expect to play some missions several times before you nail them.
Still, if you like the basic idea behind the game, and are not afraid of reading manuals, nor of spending some time to master the UI, this is a pretty engaging game--and there's nothing else quite like it available.