OSX's "top" program doesn't quite behave like its Linux counterpart out of the box. For me, the two biggest problems are that processes aren't sorted by CPU usage and the top program itself uses 10% of the CPU because it calculates all sorts of statistics about memory and shared library usage that I personally don't care about.
ReadI'm a relatively new Mac user so I'm keeping a list of some useful free apps that I'm using on my Mac.
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