A few minutes ago, my dock crashed in OS X. Everything else was fine, but I couldn't launch anything or switch between apps. Usually, people would reboot in this situation, but there's a much faster and easier way to fix a frozen dock, which meant I could continue working without having to worry about saving and restarting everything. In fact, you can fix a stuck Finder, Menubar, and Spaces as well. All you need to do is launch Terminal, and type in a simple command for each. Here's what you do after launching terminal in these situations - these are all case-sensitive:

If the Finder crashes:
killall -KILL Finder
(Or, you can right-click the Finder and select Relaunch)

If the Dock crashes and becomes unresponsive:
killall -KILL Dock

If Spaces crashes and you can't swap between them:
killall -KILL Dock

If the Menubar crashes and can't be clicked (beachball):
killall -KILL SystemUIServer

There you go! Easy ways to quickly fix system issues that might crop up on your Mac.