Android: One of the best things about Ice Cream Sandwich and Jelly Bean is the ability to long-press the home button to switch or close apps. ICS Task Manager/Switcher, the handiwork of XDA developer hamzawi, brings the same functionality to all Android devices for free.
The free version of the app 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 as well as the ability to shake to kill all open applications and start automatically when your phone boots up.
Grab the app at Google Play for free, or hit the link to the XDA thread below for more information.