Cyclists' Touring Club, a UK based organization for touring cyclists. Founded in 1878, the club now has around 69,000 members. Activities include organizing large events and rallies and lobbying local and national government on cycling matters such as road, path and transport planning; they also have a large number of contacts and affiliates worldwide for members planning to tour abroad.
Membership gives free legal aid and third-party insurance, which is very worthwhile if you do any serious amount of cycling.
CTC web site is at http://www.ctc.org.uk/