How do I switch between open windows?
Command+Tab switches between opened applications, but for example, when I open preferences-window of BetterTouchTool I don't get it shown via Command+Tab.
Cmd+` will allow you to switch between the open windows of the current application. Very useful.
By default (on a clean install of Mountain Lion), there is a keyboard shortcut "Move focus to active or next window", ^F4, which worked for activating the BetterSnapTool preference window (which is the same deal, activating a window in an app that doesn't have a dock icon).
If you want to switch between windos within ONE application, you rather want to use the "Move focus to next window" command, 3 lines below the marked line in the screenshot above.
Shortcut key to switch between opened windows is Alt+Tab