Any window in Windows can be set to be "always on top", which means that no other windows will overlay it on your screen. By using a AutoHotkey script, you can assign a global shortcut key to toggle the "always on top" flag for the active window, like this:

^+t::
      WinSet, AlwaysOnTop, Toggle,A
return

In this example, the keyboard shortcut CTRL+T sets or releases this flag.

I have had this script lying around for quite some time before figuring out an actual reason to use it: whenever I am in a conference call (using e.g. Office Communicator) I CTRL+T the window, making sure that I can quickly reach the mute/unmute button for my microphone.