Keyboard shortcut to hide/show Explorer navigation
I don't believe there is a keyboard shortcut (I just tried all the ctrl/alt + alphabet keys, and none of them opened the navigation pane)
Here is how I do it:
1) Alt+D (puts the focus in the Address bar, so works wherever it was before)
2) Tab, Tab
3) Space
4) L
5) N
This is optimized for speed of typing; you can save a Tab if you use Ctrl+E or Ctrl+F to put the focus in the search box instead.
If you've just opened the window, three presses of Tab will get you to the Organize button in step 3.