Recently I've become a bit obsessed with CPU usage. Not because my 2011 MacBook Pro is underpowered -- far from it with its Quad-core Core i7. My concern is that the CPU generates an absolute bucket load of heat.
CPU LED is a fantastic little menu bar tool for keeping an eye on your CPU load. It sits there with a small, customisable graphic that updates dynamically to display CPU load at a glance. It'll display each core or virtualised core separately or create an average for the whole CPU, depending on your preferences.
There are loads of indicators to choose from, including Apple symbols, smilies, bars, colored circles and 3D chips. If you can't find something to your taste you can easily load your own icons.
While CPU LED is a free download from the Mac App Store, there are two add-ons available as in-app purchases. The first is a Memory LED, which is available for US$0.99 and does what CPU LED does but for RAM, so you can keep an eye on your RAM usage and see when your system is paging to and from your disk. The second is Disk LED, which will set you back $1.99 (also available as a separate purchase). Disk LED monitors your disk activity and allows you to check their health status on-the-fly.
While programs like iStat Menus are more configurable and comprehensive, CPU LED gives you at-a-glance information on your CPU at all times, is lightweight and most importantly, it's free. So if you're at all concerned about your CPU load and you've got space to spare in your menu bar, give CPU LED a whirl. You might like it.