Apps that have just gone free or are going to?
There are a lot of Apps which provide a daily list of free Apps. These kind of companies base their business in agreements with the developers, who pay for their Apps to be featured on these App-of-the-Day lists.

So, the Company makes it's business, the Developer's App get notoriety and publicity, which means that on the long shot more people will download the App once it's paid again and the user gets to get free Apps daily. It's a win-win-win.

The best and easies way of knowing of paid Apps going free for limited time is using some of these Apps.