CTRL - Tab takes you to the previous tab you were on. Other keyboard shortcuts can be found here and in the Modeler menus too (see the Edit menu for some shortcuts).
CTRL + Tab and CTRL + SHIFT + Tab should do the trick.
What Mike and Robert said. CTRL-SHIFT-S is also a very handy shortcut to save all the changes in all open tabs, ctrl-s only saves the changes in your current open tab. F5 lets you run the app and F9 lets you view the app.
One thing which is a little bit different from internet browser tabbin: CTRL-TAB takes you to your previous tab. So when you relesae the keys and press CTRL-TAB again your back at the tab you started.
Is there any way to change CTRL-TAB and CTRL-SHIFT-TAB to do normal left to right tab changes, as done in Chrome and most other tabbed applications?