Sometimes, nothing beats seeing a chart to quickly see what's going on.
http://www.ragingmenace.com/software/menumeters/index.html
It's always instantly clear when something like iTunes or a Chrome helper starts eating up 105% CPU, and provides quick access to force quitting it. :)
Screenshot: http://i.imgur.com/sEtkR9p.png
I actually mostly use glances [1]. I forgot to mention glances in my comment.
[1] https://pypi.python.org/pypi/Glances
Sometimes, nothing beats seeing a chart to quickly see what's going on.