Source: http://lifehacker.com/5632748/seepu-monitors-android-cpu-and-memory-utilization
Android: Like traditional utilization meters for desktop computers, SeePU offers you a peak into your Android phone's heavy lifting with CPU, memory, and network monitoring.
SeePU puts monitoring meters in your Android phone's notification are. You can monitor the CPU usage, RAM usage, and network activity. You can adjust the frequency the monitor is updated, how the data is displayed—just CPU or CPU and RAM combined, upload and download or combine network activity, etc.—and if SeePU will start at boot.
You may not need to constantly monitor your Android phone but for those times you're trying to get the bottom of a lagging interface or abnormally high wait times, it helps to see when the load on your phone is peaking. SeePU is a free application and works wherever Android does. You can download it by searching for SeePU in the Android Market or scanning the QR code to your right.
Have a favorite tool for monitoring your Android phone? Let's hear about it in the comments. For more monitoring apps make sure to check out previously reviewed JuiceDefender and Watchdog.