I recently -- a mere two weeks ago -- discovered that the IDE lets you perform actions on multiple projects selected in the Projects window. Just use the Shift and Ctrl key together with your mouse and you too will see what I saw:
Not bad, huh. The IDE even adds up the number of projects that I've selected, as if it's boasting: "You want me to close 7 projects? You think I can't handle that? Ha. Wait and see."
And, if you liked this tip, maybe you'll also like these earlier ones:
Quickly Unclutter the NetBeans IDE Source Editor
Quickly Navigate through the NetBeans IDE Source Editor