Android: One of the great things about Ice Cream Sandwich and Jelly Bean is that you can long-press the home button to switch apps or close apps running in the background. ICS Task Manager/Switcher, the handiwork of XDA developer hamzawi, brings the same functionality to all Android devices including those that will never get ICS or Jelly Bean, for free.
ICS Task Manager/Switcher pretty much does what the name implies. There are two versions of the app: The free version just gives you the task switcher, and lets you long-press the search button to bring it up. The app has a homescreen widget to close all open applications if you’re not sure which one is slowing things down. If your phone is rooted, the app will take screenshots for you too. ICS Task Manager/Switcher can also live in your notifications pane (if you want) for quick access to the switcher, or if long-pressing search doesn’t work.
If you’re willing to drop $4, the Pro version includes all of the free features, but also lets you shake to kill all open applications, will start automatically when your phone boots up, and more. For more information, grab the app at Google Play for free.