Adventure rider primarily American adventure site - BMW GS compulsory.
Lots of content, hard to navigate.

Bikers Oracle multiple different make forums.
Some busy, some empty. Flash player needed.

BMW club UK you need to be a club member.
Great if BMWs are your thing.

Visordown UK based site - mainstream biking.
Now run by Two magazine - grow a thick skin

The RevCounter Lots of social gatherings, some even ride motorcycles!