Windows/Mac/Linux: GKrellM is a free system monitor that is highly customisable right out of the box with a host of settings, further enhanced by a large pool of diverse plugins.
Originally a Linux application, GKrellM has been ported to Windows and Mac OSX. GKrellM has a simple single stack interface that is extremely customisable. You can configure it to monitor dozens of variables on your system including CPU usage by composite or individual cores, temperatures, fan speeds, system processes, disk usage, and more. In addition to the lengthy list of things you can monitor with the default components of GKrellM, there are numerous plugins available. Plugin customisation allows you to extend the power of GKrell to do all sorts of things including updating you on the weather, tracking to-dos, monitor site uptime, rotate your wallpaper and even up keep an eye on your TiVo. You can easily replace other minor applications your your wallpaper switcher with a simple GKrellM plugin. GKrellM is freeware and available for several operating systems including Windows, Mac, and Linux. GKrellM [via MakeUseOf]