X Resource Graph
Mike Piatek-Jimenez finally released X Resource Graph 1.0, an open source system monitor for MacOS X that allows you to monitor CPU activity, memory usage, battery status, machine temperature, network activity, disk I/O, current weather, and stock market data. It's light, very flexible and highly customizable.
I've started using (and beta testing) this app about one year and half ago, now, and it immediately became one of my indispensable utilities (the kind I install on every machine I use).
Don't get fooled by the 1.0 version, BTW. It would have largely deserved to be called 2.0 or even 3.0, because of the number of features that have been added since I started using it, but Mike used a slow (3 digits) versioning scheme (the previous release was the 0.5.0), so this is a quite polished 1.0.
So if you ever need to monitor your computer's activity, give XRG a try. It's free, anyway.
Thanks for that great app, Mike!