Mac: When you press Alt+Tab on your Mac, by default you see large icons for each running application, and you can tab over to select one and switch to that app. It's fast, but in Windows you can see actual previews of each window, so you can go right to the window you want. HyperSwitch brings that same functionality to OS X.
HyperSwitch is in beta, but it does something that a lot of people have wished OS X did by default for years. If I have three or four browser windows open and I switch to another app, when I Alt+Tab back, I can only choose to bring the browser back to the front -- I can't choose which window I want to work in. HyperSwitch fixes that, and it's free. You can choose to see app icons, and the window previews only show when you highlight one of the apps (and you can arrow-down to select the window you want) or you can tell it to show you individual windows all the time.
The app is somewhat similar to previously mentioned HyperDock and is made by the same developer, but it's a little more keyboard-friendly. While it's in beta, HyperSwitch is free. You can grab it at the link below.