Any power-user knows the ⌘+Tab shortcut is indispensable. What you can't do, though, is tab between two windows in the same program. A workaround for this is to use two different browsers at once (eg. Firefox and Chrome) to tab between the two.
EDIT: Okay, scratch that — clearly I'm not the power-user I thought I was. Use ⌘+~ to switch between windows of any program. Thanks for the tip, Dura!
I also failed to mention Ctrl+Tab to switch between tabs, thanks Daniel. Lastly, check out Contexts, thanks for the tip Dahef.