iStat for Mac

From iSlayer:

iStat is the most powerful system monitoring application for Mac OS X. iStat lets you monitor every aspect of your system and features a simple yet beautiful interface. iStat's a simple yet powerful interface features fully anti-aliased graphs and a large assortment of customisation options. Change colours, transparency, window shadow and type size. Float the window above everything or pin it to your desktop. It's entirly up to you. iStat includes 8 sections that can be turned on and off to suit your needs. You can even hide specific disks, network interfaces, temperature sensors or fan sensors to ensure you're not distracted by what you don't need to see. Please note: The SpeedIt kernal extension is required to view temperatures on Intel Macs. Fan sensors on Intel Macs can not be read using iStat yet.