If you like to keep an eye on exactly what's using resources on your Mac, then System Lens is a simple and effective solution.

Quit and monitor resource usage

System Lens does a similar job to Task Manager in Windows or Activities Monitor in OS X, i.e. it makes it easy to manage which programs are running on your system at any one time. If your Mac is running slow, then a tool like System Lens can quickly fix the problem.
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Probably the most useful feature of System Lens is the ability to instantly quit applications that are causing problems. This works similar to the "Force Quit" option in activity monitor. If you use this option, remember to Save your work first unless the app is completely frozen.

Also, rather than show what percentage of your CPU an app is using, System Lens indicates whether an application is using a low, medium or high percentage of your Mac's resources. This is great for those that want a simple overview of resource usage although power users will miss the ability to see exactly how much certain apps are hogging your system.

Very simple to use

System Lens sits in your Menu Bar. When you need to see why your Mac is running slowly or what is hogging resources, just click on it and it will display a drop-down list of the applications open and how much of your CPU they are using. By clicking on the cog in the top right corner, you can filter exactly which apps you want System Lens to track.

Simple and effective resource tool

System Lens is an easy way to make sure your Mac runs smoothly and prevent applications from slowing it down.