ZZ9 Plural Z Alpha (aka ZZ9) are the official Hitchhiker's Guide To The Galaxy Appreciation Society. They were established in 1980 and have released a quarterly magazine, Mostly Harmless, full of interviews, fiction, news, art, discussion, and so on, ever since. They also produce a wide range of Merchandise related to Douglas Adams exclusively for members of ZZ9. They come highly recommended by many people on the newsgroup.
They also arrange and coordinate meetings for fans of the Hitchhiker's Guide, mainly based around the UK.
They are able to accept membership subscriptions in GB pounds or US dollars.
Their website is http://www.zz9.org