Ever find yourself having so many tabs open in your browser, that you can't see what is what anymore? I just had that and ended up going into the wrong MODX manager tab to make changes. Argh!
Luckily there's a simple way to give the MODX Manager a distinct favicon if you want to, and if you find yourself in some sort of tab-ception and can't find the right tab, I encourage you to do the same.
Simply open your (Revolution 2.1 or up) manager, and head over to System > System Settings.
In the search field in the grid toolbar, type "favicon" and hit enter. You should find the manager_favicon_url setting.
Double click in the value column to open the editor, and then add either the full URL to your favicon, or an URL relative to the manager directory. In the screenshot below you'll see I referenced an icon for the modmore site - this shows me that I'm in the live site and not my local install I was working on..
Other urls that may be a nice fit for your manager (all of these ship with Revolution):