One of the major driving forces of the Amazon Appstore — when it was first released — was the free app of the day. In the beginning, Amazon sported useful apps that if you were on the ball, saved you hundreds of dollars in purchases. However as time progress, the quality of free apps per day as since declined. However every so often, Amazon has promotions that brings back the chance of getting those useful apps for free. Starting today, Amazon is holding a promotion, through tomorrow, offering about a $100/£100 worth of apps for free.
Some notable apps include Plex, Accuweather Premium, Sonic the Hedgehog 2, and Splashtop Remote Desktop. Depending on whether you're in the UK or the US, you have a few different apps to pick from, but for the most part they're consistent. Hit the source to get to your respective deals.
source: Amazon (US), Amazon (UK)
any free app from amazon forces you to be signed into there Amazon App or the free software does work
If you love Android you should not give Amazon one Cent buying here apps. Amazon does really nothing to contribute to Android. They just want to get money out of it for free. So use the official google play store, and you will support a good future for Android and great innovations by Google.