On Android, there are plenty of ways to monitor your processor, battery, RAM usage, etc. However, there is really no way to monitor said information while continuing to do other tasks, since Android only allows one window in the foreground at a time. That's where Cool Tool, a simple resource monitoring app, comes into play.
Cool Tool is different than other resource monitors on Android because it has an option to stay on top of other windows or the notification bar, allowing you to constantly monitor your hardware statistics. While this option isn't all that practical to use one hundred percent of the time, it could definitely be a highly useful tool when troubleshooting. Perhaps the device gets laggy at random times, for no apparent reason -- just enable Cool Tool to see what's going on. Of course, it's not a full solution, as it won't show you which app is hogging resources, but it will give you a better idea of what resource is under siege.
As far as options are concerned, Cool Tool offers monitors for CPU load and frequency, RAM load, data traffic meter, battery status, and current time. It doesn't offer a whole lot in away of aesthetic customization, however, as you can only change the text and background transparency.
Cool Tool is free in the Android Market, hit the widget to grab it.