[Update: the deal/mistake is over -- the price has reverted back to $4.99]
If you're an Android owner, stop what you're doing, follow this link, and grab The Cave. It isn't a game changer in the puzzle or adventure arena in the slightest, but it's still a particularly imaginative and fun game in its own right.
This deal is available on the Amazon App Store (which is confusingly separate from Google Play), and it's not advertised as the "free app of the day" -- so make sure and grab it fast. It's still normally priced at $5 on Google Play.
The Cave [Amazon - Thanks Dave!]