iStat menus is a great way to monitor your Mac. It allows you to monitor CPU usage, memory, disk usage, CPU temperatures, fans and power, networks usage, bluetooth usage all from your menu bar. It's highly customizable. You can choose exactly what things you want to monitor. And for each thing you monitor, you can further customize it so that you can have it displayed exactly how you want it.

Once you download the package and run the installer, you won't actually see anything in the menu bar. To have things show up, you need to go to the System Preference panel. Once there, you'll see a iStats menus option in the "Other" section. Select this and you can then configure what system monitoring elements you want displayed in your menu bar, and how you want them displayed.