There are currently not one, but two ebook bundles that let you pay what you want for a bunch of science fiction ebooks. Head over to the Humble eBook Bundle and StoryBundle to grab several books for potentially little money.
Humble Bundle is running its second ebook bundle with Cory Doctorow's Little Brother, Cherie Priest's Boneshaker, Robert Charles Wilson's Spin, and Lois McMaster Bujold's Shards of Honor. Pay more than the average to grab ebook copies of Peter S. Beagle's The Last Unicorn and Wil Wheaton's Just a Geek.
StoryBundle runs various ebook bundles, and right now the focus is on science fiction with Kevin J. Anderson's Hopscotch, Michael A. Stackpole's In Hero Years...I'm Dead, Mike Resnick's Santiago, Kristine Kathryn Rusch's The Disappeared, David Farland's On My Way to Paradise, and B. V. Larson's Swarm. Pay more than the threshold amount to get Frank Herbert's High-Opp and Gregory Benford's The Stars in Shroud.
Both bundles are available for a limited time, and let you designate how much money goes to the authors. Plus, the ebooks are DRM-free.