iStat Menus lets you monitor your system right from the menubar. Included are 8 menu extras that let you monitor every aspect of your system.

Some features:
* CPU - Monitor cpu usage. 7 display modes, multiple core support.
* Memory - Monitor memory usage. 4 display modes, page ins/outs and swap usage display.
* Disks - Monitor disk usage and activity. 6 display modes, ability to hide disks you dont want to see.
* Network - Monitor current and total bandwidth, peak bandwidth, ip addresses . Ability to hide disks you dont want to see.
* Temps - Monitor the temperature of your mac. 2 display modes, ability to hide sensors you dont want to see.
* Fans - Monitor the fan speeds in your mac. 2 display modes, ability to hide sensors you dont want to see.
* Bluetooth - Control bluetooth status plus monitor the battery level of your Apple wireless keyboard or mouse.
* Date & Time - Date + time in your menubar. World clock display lets you see the time in multiple locations around the world.

What's New
Version 3.11:
* Improved sensor support for late 2010 MacBook Air's
* Improved sensor support for Intel based XServe's
* Fixed bug with temperature sensor support on MDD G4's
* Fixed bug with empty fields in the Combined extra causing incorrect display of the CPU extra in the source list
* Performance improvements

Requirements
Mac OS X 10.5 or later.

Link for more information: http://bjango.com/apps/istatmenus/