Android only: While Android has a few multitasking tricks built-in, Power Strip re-assigns your Home button to bring up a dock containing app shortcuts, folders and even widgets that you can launch at any point, without returning to the home screen.
The built-in "recent apps" window (launched by holding down the home button) is nice, but Power Strip brings fast app switching to a whole new level. By hitting the home button (or long-pressing the search button, if you prefer to leave your home button untarnished), you can bring up a customisable dock in any app. Not only can you place shortcuts and use pre-defined folders of shortcuts in this dock, but you can even bring up widgets — without ever leaving the app you're in.
For example, say you're in a music or video streaming app and want to turn Wi-Fi back on. Instead of going to your home screen and swiping to the Power Control widget, you can just bring it up with two taps, hit the Wi-Fi toggle button and get back to what you were doing right away. Or if you're browsing the web and want to skip to the next track in the music player app, you can just bring up the widget, change your song and minimise it again. It also lets you store shortcuts in the dock, so if you have a few oft-accessed apps, you can open them up immediately, without having to swipe to their shortcuts on your home screen. If you are a true multitasker, hit the link to check it out.
Power Strip is a 99 US cent download for Android phones running 2.1 and up.