FireScribe (US$0.99) is a cute app for sending something beyond a typical text message. The app creates fireworks against any background you choose, then spells out your custom message in pyrotechnics, letter by letter.
You start be selecting a background. It can be from your camera roll, Facebook, or from a gallery included with the app, or you can take a fresh photo from within the app.
Type your text, and then the app will render the video. From there it saves it to your camera roll for sending via email or Apple Messages. The videos are pretty compressed and are 12-15 seconds in length.
FireScribe for iOS
I wouldn't want to send a fireworks message on a regular basis, but they are attention getting and easy to create. Of course the app is most appropriate around certain fireworks-friendly holidays like Independence Day here in the US, but really it works anytime.
Your text sits on the standard fireworks lattice that such displays in the real world use. I'd also like to see an option to lose the lattice and just have the fireworks float unattached.
FireScribe requires iOS 6 or greater. It's not universal, but it will run on any iDevice. If you are a frequent messenger you might like it, and the people I sent some FireScribe messages to were pretty happy and surprised.