Tonight I read on slashdot that they've improved XFree86 so that you can change the refresh rate of each screen of an X display on the fly. Yep, BeOS has been able to do this* since I've owned it. Number of times I've actually used that ability? Once, just to see it.
To be fair, you can also rotate and reflect each screen. Oh, yeah; that's useful.
*I realize that BeOS doesn't run X. I mean, the ability to use different refresh rates (and resolutions) within each workspace.