There are a few online discussion groups of interest to cyclists and some online magazines also. These are the ones that we know about. If you know of others please let me know so that we can add a link.
The National Cycle Archive was established in 1990 by the Cyclists' Touring Club with funds from a bequest by Frank Whitt. The National Cycle Archive Fund is administered by the trustees of the Cycle Touring and Countryside Trust. The National Cycle Archive sees its primary aim as the preservation of records relating to cycles and cycling. The National Cycle Archive houses its archive collections at the Modern Records Centre, University of Warwick Library.
A range of other cycling hsitory sources are listed at the Modern Records Centre, University of Warwick web site; including the CTC's own archive.
Cycle-iseWM: A discussion list for those cycling in the West Midlands.
Cycling Forums: The premier online community dedicated to all things cycling and mountain biking. Covering everything from road cycling to triathlon training to picking the right gear for you, Cycling Forums has something for all levels of cyclists.
Cyclescape is an online campaigning toolkit for cycle campaign groups in the UK.
rec.bicycles newsgroups: A range of discussion groups.
City cycling - the online magazine for cycling... in the... city.
Bike Citizens - world cycle campaigning news
VeloVision magazine - a quarterly publication that covers specialised bikes, cycling as transport and human power.