Eclipse C++: How do you quickly switch between header and implementation files?

Ctrl + Tab is the default shortcut. You can change it in Window->Preferences->General->Keys - Toggle Source/Header

Ctrl + Tab.

See in menu Window --> Preferences --> "General" category (far left) --> Keys --> "Toggle Source/Header"

Ctrl + Tab is the default shortcut and here is a list of some of the more useful shortcut keys in Eclipse.