Optimal Layout is a great utility that fundamentally alters the way that I interact with my Mac. In short, it gives me the ability to use my keyboard to quickly change between windows.
That might sound like what you're been doing with ⌘ + Tab, but there are a few important differences. The first is that you can choose not only which app you are switching to, but also which window in that app. Optimal Layout can even switch you to specific browser tabs in Safari or Google Chrome.
You can use the mouse with Optimal Layout, but it works especially well for those who prefer the keyboard. Type ⌘ + Option/Alt + F and it will bring up a list of available windows. Start typing and it will match window names. When you find the one you want, simply press Return. This is especially great if you are on a Mac with a small screen or working in multiple full-screen apps, because you can switch between them without using ⌘ + Tab to find the app you want to use.
Optimal Layout is more than just an app/window switcher, it also lets you control the location of windows on the screen with your keyboard. You can maximize (full screen minus the Dock and menu bar), half-screen, quarter-screen, or center any window via the Optimal Layout UI, without bringing that application forward. Just type to find the window you want, and then press the keyboard command for where you want that window to go. Or you can use the menu bar controls (which can also be hidden, if you don't want them to appear) to control the front-most window. If that's not enough, there is even a grid that Optimal Layout can show you and help you position all of your windows on your screen if you need to see multiple windows at once.
The best way to understand Optimal Layout is to see it in action and try it yourself. There is a 94-second screencast on the Optimal Layout homepage, and a few other short screencasts on the support page. There's even a trial version available so you can try before you buy.