Android/iOS: If you struggle to fall asleep sometimes, check out this free app. MySleepButton helps knock you out by making you imagine random images.
Select how much time you want the app to play, and click the Put Me to Sleep button. A woman will say random words, such as "jaguar" and "pencil," and you're supposed to image each one. This keeps your mind off of other thoughts that could distract you, and it's surprisingly effective at making your mind tired and into a sleep-induced state. The app was developed by a cognitive science researcher, Dr. Beaudoin of Simon Fraser University.
Other options in the app are to imagine scenes (e.g., "bird on a branch"), imagine yourself drawing various items, and change the voice. Some options are only available as in-app purchases ranging from $US3 ($4) to $US5 ($7), but the basic app is free.