A while back, we covered a jailbreak tweak called Grafiti [sic]. It allowed you to instantly create and share drawings via the messages app, by acting as a keyboard replacement. WIth that in mind, it supported a lot more apps than just the stock Messages app; it worked with Whatsapp, Mail, Tweetbot, and more.
Doodle Message is similar to Grafiti, in that it allows you to create drawings from within the Messages app, and send those drawings to your friends via Messages. The big difference here is that Doodle Message only works with the Messages app. There are, of course, other smaller differences, as we'll touch on inside.