Changelog Droid, an app that not only shows changelogs of applications you have installed all in one convenient place but also monitors apps that you haven't installed and keeps history of changes over time, is on sale for 24 hours. And by sale I mean it'll cost you about free fifty. I've played around with the app for the last half an hour and found it to be very polished, pleasant to use, and, more importantly, actually handy. It even has a DashClock extension.
Since the Play Store doesn't allow apps to switch from free to paid, the developer is using the next best method - a coupon code.