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.
Take a look at some of the 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.
Link: iStat Menus