Arcade BBS
The definitive Acorn based bulletin board, on the net. It's on-line in south-east England, practically all the time for access by modem dialup, serving as a virtual meeting point for hundreds of users and holding an impressive array of files numbering a few thousand.
CCl4 - Solvent Abuse
Bizarre BBS which seems to have some weird connection to RISC OS.
The Plasma Sphere BBS
Supporting speeds 300-33600 bps, two lines, online 24 hours a day, 365 days a year. Online support for Acorn, ArmBBS, FidoNet, Arc-Binkley, and ARCbbs in its 8500+ file filebase and message base.