Stars Without Number is probably the best-known D20 based sci-fi ruleset published, even more so than White Star. While WS builds off of the "Original White Box" / S&W White Box for its engine, SWN is something a bit different.
I've run SWN and it plays very smoothly. It is built for sandbox style play with rules and tables that solidly support such.
If you want, you can check out the Stars Without Number Revised rules for free. (link)
There are two Stars Without Number offerings currently on Bundle of Holding:
For just US$8.95 you get all three titles in this offer's Stars Without Number Revised Starter Collection (retail value $45) as DRM-free .PDF ebooks:
There is also:
And if you pay more than this offer's threshold price of $17.12, you'll level up and also get this offer's entire Bonus Collection with four more sourcebooks worth an additional $35, including Starvation Cheap (wars and mercenaries), Dead Names (ancient civilizations), Sixteen Stars (popular sf settings), and Polychrome (cyberpunk).
- Stars Without Number Revised Deluxe Edition: The premium full-color 324-page rulebook with 80 pages of bonus content covering transhuman tech, true AIs, mechs, society creation, and even Space Magic.
- Other Dust: Kevin Crawford's 208-page post-apocalypse sandbox RPG completely compatible with Stars Without Number. Includes the freebies Grandfather's Rain, Cult of the Wraith, and Cult of the Still Lady. (Previously offered in the original October 2013 SWN Classic collection.)
- SWN Revised GM Screen: A portrait screen with eight pages of useful reference charts.
For just US$7.95 you get all three titles in this offer's Stars Without Number Classic Starter Collection (retail value $35) as DRM-free .PDF ebooks:
And if you pay more than this offer's threshold price of $17.15, you'll level up and also get this offer's entire Bonus Collection with four more titles worth an additional $34:
- Stars Without Number Classic: The complete original 253-page First Edition rulebook (2011).
- Skyward Steel: Join the Thousand Lights of the Stellar Navy and embrace both the duties and the terrible freedom of a naval campaign.
- Hard Light: The first SWN adventure setting. In the radioactive glare of a red giant star, explore the hard-bitten mining outpost of Brightside and ancient asteroid Sky Tombs.
- Suns of Gold: Merchant campaigns and far-flung trade, including trade world tags and guidelines for building your own colony worlds and merchant empires.
- Engines of Babylon: Create vehicles and low-tech system spaceships for your SWN campaign. This equipment book debuted in this offer's July 2014 revival.
- Relics of the Lost: A sourcebook of long-dead technology from the Second Wave of human colonization. This supplement debuted in this offer's original October 2013 run.
- Darkness Visible: Spacefaring espionage campaigns of classified perils, nameless heroes, and secret glory.