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, and the ability to hide network connections you don't want to see.
    Temps -- Monitor the temperature of your Mac. 2 display modes, ability to hide sensors you don't want to see.
    Fans -- Monitor the fan speeds in your Mac. 2 display modes, ability to hide sensors you don't want to see.
    Bluetooth -- Control bluetooth status plus monitor the battery level of your Apple wireless keyboard or mouse.
    Date & Time -- Date and time in your menubar. World clock display lets you see the time in multiple locations around the world.

Requires OS X 10.8 or later

Home Page - http://bjango.com/mac/istatmenus/