Feel the Wheel (Feewhee) is a tool that Microsoft apparently overlooked while making Windows. By using Feewhee you can easily resize windows and control their transparency with a mouse wheel.
Since the first Feewhee's quiet release back in 2005, such functionality became popular. It was reproduced in some windows managers and copied by a dozen of similar utilities. Yet nowadays Feewhee is probably still the simplest, most natural and unobtrusive.