Mac: Got an app or two running down your battery? Never really sure what it is? System Lens pays attention to active processes and how much they utilise the CPU. It warns you of the greedy ones so you can keep an eye on what's killing your battery.
System Lens lives in the menu bar and shows you high, medium and low processes currently active. If you want to kill one, you can click on it to get an X button and stop it from utilising your CPU. The app profiles your CPU usage regularly, so it obviously requires some of its own, but you can disable intermittent polling to save a few little cycles and a sliver of battery.
The app also offers customisation options so you can set what type of activity determines low, medium and high usage. It's like a mini, menu bar-based, more-focused Activity Monitor that's definitely worth a download if you need to keep an eye on what your computer's up to.
System Lens (Free) [Mac App Store]