iStat menus is a very customizable menu bar application. It allows you to monitor temperature, fan speeds, data rates, RAM usage, CPU usage and more. Its crowning glory is the ability to customize your date and time to include a pull-down calendar and world clocks. The application is donationware so give it a look. Mac only.
[Shout out to my brother-in-law for the tip.Yep, my whole family is Mac. Totally rocks.]