www.opelgt.com
The most active Opel resource in North America, it features a combination of sometimes folksy discussion with technical suggestions and parts advice. Some features require online registration for access.

http://a.webring.com/hub?ring=opelring
The "Opel Ring" features a random listing of Opel-related websites, some featuring personal information others featuring various Opel car models sold elsewhere than the North American market.

www.opel.de
Official Opel factory website from Russelsheim, German. Text language is German, and requires Macromedia Flash 7 program download for front page displays.

http://autos.groups.yahoo.com/group/classicopels/
Another active Opel resource, is the various Opel message boards at "Yahoo Groups" including the most-active "Classic Opels" group. Registration through Yahoo is required to join.

www.ebay.com
A search of  "Opel GT" "Opel Manta" "Opel Kadett" typically lists cars and parts for sale. (Note: Not all sellers are knowledgeable or accurate, so Please exercise caution in all purchases for Opel items).