I routinely Alt-Tab between my current 2 or 3 windows when working, but i often have 20+ windows open at a time across 2 monitors and often times I will SEE the window I want right there, but selecting it is nigh on impossible.

One of the biggest culprits is MS Office (especially Excel, where I routinely keep data i need to refer to every now and then). Unless the title bar of the window is visible, clicking on any other edge will change the current active tab, pull down a menu, open a dialog, disable recalculate mode or some other weird behavior that is sometimes not readily apparent.