On: Poll: What OS do you use everyday, or most often?

[jodrellblank]

There's an app called KatMouse for Windows which might be able to do focus follows mouse.

(I really use it because it lets me use the scrollwheel on any window, without bringing the window to the foreground or giving focus to it).

[Zancarius → jodrellblank]

I'm not so sure about Vista/Win7 but Windows XP's tweakui allows a primitive form of "focus follows mouse." It doesn't work exactly like I'm used to with other managers, but it also doesn't require loading additional software (well, beyond going to MS' site and downloading tweakui).