An activity monitor in the menu bar!
I have to say that in terms of easy access activity monitor, iStat Pro was enough for me. That was until I tried iStat menus. The interface is entirely customizable in System Preferences and you always get a quick preview without having to launch the dashboard. Another great thing is the Date & Time menu. You can even load iCal from the Date & Time menu. More info and download link here on the developer's website.