Feewhee lets you instantly resize windows and make them transparent
Weighing in at a massive 4 kilobytes for the portable version, Feewhee does exactly two things, and does them well:
First, get the program and run it. Then, hover your mouse over the title bar of any window. Now scroll the wheel, and watch the window resize up or down, according to the direction of your scroll.
Press shift and scroll, and watch the window gradually become less opaque (or more transparent, in other words).
I love this kind of software because it's tiny, free, portable, does exactly one thing and does it exceptionally well. I'm more of a keyboard kind of guy so I'm not sure how much use this would get on my system, but at this size (consuming 866KB of memory while it's running) I really don't mind having it sit there in the background, just in case.
By the way, while you're there, you should really check out the entire Nattyware site. They have some very nice freeware utilities, other than Feewhee.