Android: If you leave yourself sticky notes for all-day reminders, Toastr is a small app that can move those reminders to your phone. Just tell it what you want to remember, and it will give you a small, customisable popup every time you unlock it.
That's pretty much all there is to Toastr -- it's simple, useful and effective. Just tell it your message and whether you want it to last 1.5 seconds or three seconds, and you're done. The $0.99 pro version also lets you customise things like the position of the popup, size and colour of your text, if you want to get a little more creative.
Toastr is a free download for Android only, with a $0.99 pro version available.