Late answer for those who still find this search result:
Alt+Tab is default for switch between apps ignoring windows. You can wait a bit and thumbnails of actual windows will come up, nice for completely new users but not very useful as speed is the whole point of keyboard shortcuts.
But actually a quick Alt+Tab and release will always switch between the top-most 2 previous selected windows, even if they are of the same app. This is a more or less insane behaviour when you spell it out like this, but in practice it is nice for old habits.
Alt+~(above Tab) is default for switch between windows of same app only. A little awkward unless you are a Mac power user or something. But what is very nice about this is that you can hold Alt and use Tab or key above at will. Which is to say you can Tab to drill down to the app you know you want and then key above to the specific window without ever releasing Alt until you are done.
More good options have come on board over time (both in 13.04 I think). They are not keyboard shortcuts but are really good for exploring everything that is open if you have gotten very busy or cluttered:
- you can right-click any launcher tile and see windows only of that app are in that list.
- you can scroll the mouse wheel over any launcher tile to immediately switch between windows only of that app.
Keyboard shortcuts are not really ideal for this because they are instead for when you know exactly what you want to switch to.