SidecarJohn

http://www.advrider.com/ - Not totally sidecar, but the Adventure Rider site has a very broad ranging Hacks (sidecars in USA speak) forum, which could be useful.

http://www.hpsidecar.com/home/ - US high performance sidecars, includes a forum.

http://www.sidecar.com - Site of the U.S.A. - United Sidecar Association, and the Sidecar Industry Council. now run by enthusiastic owners.

http://www.sidecar.it - Site of the Sidecar Museum in Cingoli, Italy.

http://www.sidecar-cz.com - Interesting Czech site.

http://www.warkshop.com - This guy paints bikes, but there's a heavy emphasis on sidecars. Some good images.