At this point, most of us here at AP have retired the Amazon Appstore. If you still use Amazon's Android app distribution software, though (read: anyone who owns a Kind Fire/HD), then today's free app of the day – Osmos HD – is actually worth grabbing.
For those who may not be familiar with the title, it's a quirky physics-based puzzler where you control simple one-celled organisms (called "motes") and absorb smaller organisms. It sounds simple, but it's actually quite challenging.
Osmos is actually a great, soothing time-waster, as the ambient environments and soundtrack easily invoke a zen-like state of relaxation. And for the low price of free, it's really hard to beat. Hit the link to grab it.