Duke Nukem 3D, the good ol'-fashioned 1991-style Android FPS, is quite the game: sure, the plot is focused on invasion by sex-crazed aliens, but as the developers themselves said in the app's Market description, "that's what makes it fun!"
And for the next two days, the game is completely free, down from $0.99! Of course, if you want to try out episodes 2 and 3, you'll still have to cough up an additional $0.99 a pop - but still, it's not a bad deal considering the game's rather impressive quality.
Better still, this version of Duke Nukem 3D is free not only of charge, but also of ads - and you won't have to jump through any hoops to obtain it, either; simply scan the barcode below or click the Market link to get started.
How the developer is planning to end the two-day sale is unclear at the moment - once an app is marked free in the Android Market, it can't go back to paid. If they kill this listing and upload a new one, those who have already downloaded the game for $0.99 are going to be quite upset. It's possible that they'll just make the first level an in-app purchase like the others, however - that seems to be the only viable solution here.