Other Space Operas and similar projects
Colony Ship - midsize
Image from Anders Sandberg

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

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

Mars and outer planets trading post

Transhuman Space