iPulse
iPulse is a system monitoring utility, that does things graphically. At first glance, iPulse will make no sense to you in what it is displaying, but after a short while of studying a little diagram in the Preferences, iPulse will become your new favorite app. iPulse is also very customizable, with many jackets, or skins, to choose from. However, iPulss costs $12.95.
iStat
iStat Application is the free alternative to iPulse. However, it is bulkier to have on the desktop, but that makes it easier to figure out what information it is displaying.
iStat Pro is the little brother that iStat Application was built from. iStat Pro offeres system monitoring from the Dashboard.
But, if you do not want a widget that is very large, try iStat nano. It offers advanced system monitoring in a tiny package.
iStat Menus
iStat Menus is system monitoring for your menu bar. You can monitor virtually every aspect of your Mac including the CPU, RAM, hard drive, network, fans, temperatures, Bluetooth, and the date and time. Each display is highly customizable to suit your taste and needs.