What keyboard shortcut is there to organize C# usings in Visual Studio?

Since VS2017, it's a builtin shortcut. Simply press Ctrl + R, Ctrl + G. Credit goes to Emanuel Ve, who mentioned this first in the comments; I'm putting the advice into an answer for greater visibility.

Go to Tools => Options => Environment => Keyboard

Enter the key combination you want to use (click the Press shortcut keys: textbox, press your key combo as you would execute it while editing)

Then type "usings" in the Show commands containing: textbox

And now you can assign whichever version of these remove/sort usings commands that you like to the key combination.

This works for pretty much every command in Visual Studio, and from any addons you have installed. Just type a word involved in the command name to find the command. Assign away!

In addition to @Will's answer, Productivity Power Tools is a great addon that has all this - and lots lots more!