KatMouse lets you scroll through windows without selecting them first. This means you can keep your keyboard focus on a given window while taking little mouse side trips all through the other open windows. It may not sound like much, but if you IM or e-mail with a browser window open, it can save your clicking muscles a lot of work.
Another nifty feature enabled at default lets you send a window to the bottom of the heap. Just click the scroll wheel, and it banishes the window you're on (you don't have to click to select it) to the back of the queue. When you have a big window blocking others, this is much faster than clicking around or using the taskbar to bring up other programs.
KatMouse doesn't work with every program---notably, Microsoft Office 2000 programs--but it did work with the browsers and other software I tried. And when it works, the rest it gives your aching fingers is a beautiful thing.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.--Laura Blackwell