Even though I’ve been using a Windows Phone 7.5 for a while, it took me a while to find a fast way to switch between apps. Initially, I just tapped the Back button until I got to the app I wanted to switch to. This works but it is not a fast way to switch between apps. There had to be a better method. Luckily, one day, I discovered it by pure accident. And I loved it! It works just like the Alt-Tab in Windows.
The Back Button Does the Magic
In Windows Phone, the Back button is key to switching between apps. Apparently, it is also key to accessing the Alt-Tab like functionality existing in Windows Phone.

Press and hold Back until the current app is zoomed out and you see a list with all the open apps and menus on your Windows Phone.
Slide left or right until you find the app you want to switch to.

Tap the preview of the app you want to switch to and you are done. If you want to see a video demo of how this works, check the video embedded below.
Conclusion
It is a shame this tip is not very well known, nor highlighted by Microsoft via its official communication channels. I’m curious to learn - did you know about this functionality? How long it took you to discover it?