iStat Menus will put valuable resource information in your menu. I only have it showing CPU, memory, and Bandwidth usage, but it also has the ability to show drive usage, temperatures, fan speeds, date and time with more in depth calendar, etc.
![istatmenu My iStat Menu configuration.](http://b.snapfizzle.com/wp-content/uploads/2009/04/istatmenu-300x24.jpg)
The configuration screen is pretty simple to decipher and you can check for updates from here as well.
![istatconfig Configuration screen in System Preferences.](http://b.snapfizzle.com/wp-content/uploads/2009/04/istatconfig-300x242.jpg)
Product Type: Freeware (donations accepted by developer).