Manifold Press has four of their titles free on ARe and Smashwords (and coming soon to Amazon.)
From their website: The free books are Adam Fitzroy‘s DEAR MISTER PRESIDENT, Jane Elliot‘s END OF THE TRAIL, Chris Quinton‘s FOOL’S ERRAND and Julie Bozza‘s HOMOSAPIEN, all of which have been consistently popular and have made us a lot of friends. We’re sending them out free of charge now in the hopes of introducing new readers to our work, and we hope existing readers will also take this opportunity to try out the work of a different author or catch up on a book they may previously have missed.
The links above will take you to the story descriptions with "buy" links. It doesn't mention exactly how long these will be free for, but it does say "a limited period."