Watchdog Task Manager
When I obtained my first Android phone, I downloaded a task killer from the Android Market, thinking it would do wonders for my phone's performance. Au contraire. It managed to mess up the phone pretty badly, so I started doing some research: It turns out that not only does Google think task killers are bad, but so does virtually every Android developer. Such programs are so notorious for causing significant problems that many major developers won't even read your error reports if you use a task killer. Watchdog ($3.49 full; free Lite version), however, is an entirely different animal.
The first and biggest difference is that the developer of Watchdog understands Android and how the OS is designed to work. Namely, it's okay if your RAM appears to be almost entirely in use. Android's native task manager keeps it that way so you can quickly return to an application you recently switched away from. That does not mean that it's using up your phone's resources, though; it's just sitting in solid-state memory, and if Android needs to free up RAM, it will close out of some stuff to accommodate.
What slows your phone down and drains the battery is when a process hogs the processor. Sometimes that just happens--you know about it, and it's fine. Problems arise when an app starts behaving badly and becomes a "runaway process," taking up a ton of CPU power when it shouldn't be. That's exactly what Watchdog watches for.
Watchdog allows you to set thresholds for processes, and lets you set the time interval between system checks. If an app process spikes above the threshold for an extended period of time, Watchdog will alert you and then permit you to decide what to do. From the alert, you have WhiteList, Ignore, and Kill options.
Select WhiteList when you know why a spike occurred, and it's okay; doing so puts that particular app on an ignore list so you won't get any more warnings about it. Ignore disregards the specific incident, and Kill, of course, halts the process. (Note: It can't kill certain processes that are built into the Android OS.)
Amusingly, every time I powered up my phone, Watchdog alerted me that Watchdog was above the threshold. This happened only during startup, and was likely a by-product of a full system check. I just added Watchdog to my WhiteList so that I wouldn't get those alerts anymore.
At the click of a button, the utility shows real-time CPU usage, which can be handy if your phone is acting slow and you can't figure out why. You'll also find a Stats page, which allows you to peruse details such as Memory usage (Free/Total) and CPU Information.
Watchdog works very well, bringing a more-sophisticated "scalpel" approach to task management as opposed to the "dynamite" technique that task killers use. The irony is that running an application to check your system resources uses your system resources.
Fortunately, unlike some apps that run constantly in the background, Watchdog is not a battery hog. (Just make sure you don't have the interval set too short, or it will perform checks too often and cost you some juice.) It's encouraging to see a developer that really understands the inner workings of the Android OS employ that knowledge to make a truly useful app.