Mouse Options Panel
The Mouse Options panel lets you activate advanced system tweaks for mouse:
Activate window on mouse hover check box - mark it to enable the automatic activation of windows just by placing the mouse pointer over them (i.e. you don't have to click on a window to make it active). This feature has some related options:
Bring activated window to front check box - mark it to not only activate the window but also to put it in front of other windows; otherwise, the window becomes active but remains on the same position in Z-order.