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 more...

WHAT'S NEW
Version 3.19:
Final release for Mac OS X 10.5 and PPC. Future releases will require an Intel based Mac running Mac OS X 10.6 or newer.
Fixed bug that could cause GPU stats to stop updating.
Recovery partition in Mac OS X 10.7 is now hidden in the Disks extra.
Improved sensor support for Mid 2011 iMa*censored*

REQUIREMENTS
PPC / Intel, Mac OS X 10.5 or later