When I was in Tokyo, on the very crowded subway, I really needed to communicate with my sister. She was only a few meters away, but I couldn't get to her. Too many japanese in the way, and shouting wasn't exactly an option in that cramped space.
If only, I thought, there was an app that let you quickly and easily print a message on your phone. Then I could just hold it up for her to read.
Long story short: I built it. Its named Simple Prompter, and it's awesome. Simply enter your message, set size and scrolling speed if you wish, and hit GO! Presto: a simple prompted message. Thank you, Simple Prompter!
I've found it very useful, and so has a few others. If you want to buy it (and sponsor me a small bit) it's available on the app store.