From MacTake.com

iStat Menus lets you monitor your system right from the menubar. Included are 8 menu extras that let you monitor every aspect of your system.
Some features:

- CPU -- Monitor cpu usage. 7 display modes, multiple core support.
- Memory -- Monitor memory usage. 4 display modes, page ins/outs and swap usage display.
- Disks -- Monitor disk usage and activity. 6 display modes, ability to hide disks you dont want to see.
- Network -- Monitor current and total bandwidth, peak bandwidth, IP addresses, and the ability to hide network connections you don't want to see.
- Temps -- Monitor the temperature of your Mac. 2 display modes, ability to hide sensors you don't want to see.
- Fans -- Monitor the fan speeds in your Mac. 2 display modes, ability to hide sensors you don't want to see.
- Bluetooth -- Control bluetooth status plus monitor the battery level of your Apple wireless keyboard or mouse.
- Date & Time -- Date and time in your menubar. World clock display lets you see the time in multiple locations around the world.



WHAT'S NEW
Version 5.10:

- Improved performance and reduced memory usage.
- Improved support for 5k iMac.
- Added support for 2015 MacBook Airs.
- Added support for 2015 13-inch MacBook Pro.
- Added ability to have different skin settings for light and dark menu bar modes in Yosemite.
- Added more memory details when using Memory Pressure mode.
- Added an option to show activity for individual disks in the menubar.
- Added an option to show activity for each disk in a fusion drive in the disks dropdown.
- Added ability to rename cities.
- Added an option to only show MB/s or Mb/s for bandwidth in the menubar.
- Added new hardware info to network connection submenus.
- Fixed some issues where disk activity was missing for some disks.
- Fixed an issue with disk activity not working with SoftRAID disks.
- Fixed an issue where disks could show 0KB free.
- Fixed an issue with 12 hour time formats if system is set to 24 hour time.
- Fixed an issue in the network extra where the primary interface setting may not have been working correctly in the menubar.
- Improved per app network monitoring on Yosemite.
- Improved GPU monitoring.
- Improved disk activity monitoring.
- Improved layout in disks dropdown when a disk has multiple partitions.
- Improved display of some menubar text modes in Yosemite.
- Improved event dots in calendar.
- Improved moon item display when using Simple city layout.
- Improved city database