PL/SQL Developer is a great tool, but I can't get used to its Ctrl-Tabbing behaviour.
What I believe most people are used to, Ctrl-Tab should work like Alt-Tab in Windows. A history of last visited windows is used based on what happens when you RELEASE the Alt key. The current window should then be put in the top of the internal history, pushing down the others. Pressing Alt-Tab next time, takes you to the second item in the history. Pressing Alt-Tab without releasing the Alt key traverses the history from the second item and down.
In PL/SQL Developer, the history seems to be based upon what window you click on with the mouse in the windows list (I use v6.0.4) and uses this history when ctrl-tab is pressed. There is no way of altering the traverse order without using the mouse.