Worldbuilding projects on the Net, current and historical
BigIdeasGrandVision A space opera scenario written by Anders Sandberg and others in 1999, which directly inspired OA, and several of the worlds from BIGV were adapted for Orion's Arm
Beacons in the Dark - a Space Opera worldbuilding project, by OA contributor Lilly Harper
The Starmoth Universe a socialist, solarpunk world building and fiction project by Isilanka
The Epiphany of Gliese 581 by Fernando Borretti A linguist, a chemist, and a comparative psychologist explore the ruins of a dead superintelligence. (26K word story, heavily inspired by Orion's Arm.)
Vergeworlds by OA contributor Dr. Luke Campbell
Galactic Association of Intelligent Life The Hamilton Institute of Exopaleontology (archived material) by Aaron Hamilton; another project which inspired OA, and parts of which were adapted for the Orion's Arm scenario
Ad Astra by Richard Baker and D. Dye. Richard was an early contibutor to OA, and this site was another inspiration for OA.
Eclipse Phase A prominent transhumanist RPG
Mars and outer planets trading post Transhuman Space A prominent transhumanist RPG by Steve Jackson Games, Partially written by OA contributor Anders Sandberg