MenuMeters for OS X El Capitan 10.11 (and later)
What's this?
It's a port to OS X El Capitan 10.11 of our beloved utility MenuMeters by Alex Harper, at http://ragingmenace.com. As he has not yet released the port to OS X 10.11 El Capitan, I made a minimal modification to the source code he has kindly made public so that it runs fine.
Setting up
  1. Please download the preference pane from this download link.
  2. Double click the resulting preference pane.
  3. If you already have an older version of MenuMeters, the System Preference will ask you if you want to replace it. Please answer yes.
  4. The System Preference might complain that it cannot be loaded. In that case please quit the System Preference once, and relaunch it. It should work.
  5. You might also need to uncheck and then check the various meters to re-activate them, once the new version of the MenuMeters preference pane is open.
Source code
It's available at GitHub.
Contact
Please report any issue at "issues" at GitHub. If you don't know how to use "issues" at GitHub, you can email me at yuji.tachikawa at ipmu dot jp.
Why doesn't the original work on El Capitan?

As you very well know and is shown in the screenshot above, there can be various utilities put on the right hand side of the menu bar. There are in fact two types of such menu bar items, one known as NSMenuExtra's and another known as NSStatusItem's. The former are loaded and displayed by SystemUIServer, a process provided by the system. The latter can be displayed by any app written by any developer. One good thing about the former is that you can rearrange them by ⌘-dragging the menu items. I have no idea why ⌘-dragging is not provided for the latter by the system.

Anyway, due to this better behavior of NSMenuExtra's, people often wanted to write their own. In fact until and including OS X 10.1, Apple allowed it. But since 10.2, Apple had a code that blocked SystemUIServer to load non-system-provided NSMenuExtra's. But until Yosemite, there was a known way to work around it, available as an open-source code as MenuCracker. MenuMeters used this to inject their own NSMenuExtra's to SystemUIServer; in fact MenuMeters' author is one of the main authors of MenuCracker.

Essentially, until Yosemite, SystemUIServer had a fixed list of allowed NSMenuExtras. MenuCracker was an NSMenuExtra that pretended to be one of those allowed ones, which, once loaded inside SystemUIServer, removed these checks, so that more NSMenuExtras can be loaded without any problem. In El Capitan, Apple added a more stringent check of the allowed NSMenuExtra's, and MenuCracker no longer works.

So, how did I port MenuMeters to El Capitan, then? Well, I just gave up having ⌘-dragging. Then all I had to do was to, basically speaking, replace the occurrences of "NSMenuExtra" by "NSStatusItem", since the two APIs are almost the same.