An essential site to visit is A to B Magazine, where you will find, amongst other things, a comprehensive list of folding bicycles and lots of information relating to their use.
There is a much more comprehensive list of folder manufacturers and models than we have here on Chop Rossiter's web site
Birdy friends is a site in German with a lot of information and news for Birdy enthusiasts -
Die Bromptonauten, a German site with lots of information, news, tips and forum.
Brompton Forum is a Francophone Brompton enthusiasts forum. Check their organised rides here.
The Moulton Bicycle Club - lots of information for owners of all models of Moulton bicycle. Although Moultons do not fold, almost all the current models can separate into two parts for storage and transport
Brompton-Talk emailing list - discussion between Brompton owners and enthusiasts.
Moulton Bicycle emailing list - discussion between Moulton owners and enthusiasts
Tony Hadland - the well known author of books on Moultons, Sturmey-Archer gears, bicycle design, and folding bicycles ("It's in the bag!"). Lots of useful information can be found on this site.
The 'classic' Moulton - Paul Grogan's excellent guide to the catalogue specifications (and known variations) for the UK Moultons of 1963-1974, and includes in appendices some useful service information in the form of reprints of the Masterclass articles which were originally published in The Moultoneer.