RiscOs.com: the owners & producers of the OS riscoscom
RiscOs.org: community site riscosorg
Drobe Launchpad: good news site drobelaunchpad
IconBar: updated more infrequently but good coverage: iconbar
Acorn Arcade: gaming site acornarcade
If you don't know what an Acorn is, you probably won't be interested, but go read RiscOs.org for an introduction.