If you have them side by side, and you have many of them tiled, I guess you could Alt-Tab through them, but I find it easier to click.

If I'm clicking between three windows and I have four open, do you really think it's quicker to Alt-Tab? On the Mac I just hit F3 which is Mission Control which I wish Windows had (it's basically a more elegant Alt-Tab; we have ⌘+Tab but that's only icons, same thing but not as pretty).