There are many on-line resources of various types related to open relationships. This list will be updated over time, and feel free to submit your suggestions too. Loving More has been supporting polyamory and relationship choice since 1985. The blog has detailed, interesting articles posted regularly, and the website has a plethora of relevant information.
Not Your Mother's Playground is a website all about open relationships by Samantha Fraser, who is also working on a book by the same name. Samantha's writing is insightful, entertaining and educational, and she is not afraid to share many details from her own non-monogamous experiences.
Polyamory in the News is just that: an amalgamation of items appearing in the news related to polyamory. Alan does more than just post links to articles--he offers interesting commentary, based on his own research and/or interviews.
Alan's List of Polyamory Events is a companion site to Polyamory in the News, and maintains a listing of upcoming polyamory-related conferences, meetings, and festivals.
Openly Poly is a fascinating collection of stories of people who have or are trying to come out as being in an open relationship.
Poly Weekly is a weekly podcast that covers "responsible non-monogamy from a kink-friendly, pansexual point of view".
Practical Polyamory is a blog by Anita Wagner that covers a wide variety of polyamory-related topics.
The Polyamory.com Forum is a discussion forum where polyamory-related topics are discussed.
The alt.polyamory newsgroup maintains a website with links to a number of polyamory-related resources.
The Canadian Polyamory Advocacy Association has articles, information and a discussion forum.
Modern Poly "exists to create and maintain a safe and supportive space where those engaged in the practice of polyamory can come together to share ideas, experiences, resources and activism..."
The Polyamory Group Directory is a wiki-style listing of polyamory-related groups around the world.
The National Coalition for Sexual Freedom has a comprehensive site on sound bites for the polyamory community.
The Polyamory Leadership Network is "a loose association of about 100 people around the world working to advance public awareness of polyamory".
The Polyamory Media Association connects the media with media-friendly polyamorous people, and also helps those polyamorous people with how best to deal with the media.
Life on the Swingset is an excellent non-monogamy resource. They run two entertaining and informative podcasts: "Life on the Swingset: the podcast" covers swinging, open relationships, and polyamory, while "Pedestrian Polyamory" focuses on polyamory.
Poly Friendly Professionals is is a listing of professionals who are sensitive to the unique needs of polyamorous clientele. This includes therapists, medical professionals, lawyers, religious figures, and more.