As iamelton said, Android's in-built task managing system does a good job of managing what apps are using the processor, memory and battery. Task manager apps don't really do all that much useful, and can sometimes be counterproductive if you're trying to use them to save battery. Lifehacker has a really great article on this called Android Task Killers Explained: What They Do and Why You Shouldn’t Use Them (I can't link because I don't have enough posts, but the article's a search away and is really worth reading). It runs through why it's generally best to avoid task manager/killer apps, plus it also explains how Android uses RAM - in a similar way to Windows Vista/7 (basically to speed up launching or re-opening apps).
It's probably worth noting that Android already has a function to force stop apps. If you go to Settings > About phone > Battery use, you'll get a list of what's been using the battery since it was last unplugged. Some widgets like Switch Pro have shortcuts to Battery use. It's useful if you've got Maps or a launcher or a rogue app sucking up battery in the background. As a rough idea, you may have an issue if anything not a system process is using more than 10%.
If you do need to force close an app, Battery use is generally the best way to go about it.