A list of 404 world-wide centers is available from the Open Directory.
World Buddhist Directory has a huge database of Buddhist centers.
Ancient Dragon Zen Gate: a Soto Zen center based in Chicago and led by Taigen Dan Leighton, Dharma heir of Tenshin Reb Anderson in the Shunryu Suzuki lineage. You'll find links to Dharma talks (including audio) here.
Being Without Self: the website of Jeff Shore, the lay successor of Keido Fukushima Roshi. Has a number of talks in PDF format and as MP3's. Seems to be based in Europe.
Black Moon Zendo: A New York zendo led by Kyudo Nakagawa Roshi, with some nice basic writings about Zen. Read the interview with Nakagawa Roshi.
Boundless Way Zen: centre led by James Ishmael Ford, teacher authorized by John Tarrant. Site has teishos by Ford, teachings and instructions, as well as photos and other materials.
Chicago Zen Center: Philip Kaplau was the founding teacher and the Chicago center maintains close ties with the Rochester Zen Center. Today, the abbot is Yusan Graham.
Chung Tai Zen Center of Sunnyvale, California, has a series of lectures on the old Chinese Zen masters by Ven Jian Hu available for downloading as .pdf documents. Also available are translations of some sutras and some dharma lectures.
Engaged Zen Center: Based in Las Cruces New Mexico , the Order of Clear Mind Zen practices street Zen, a socially engaged practice of bearing witness to war, poverty, and ecological issues practices and teaches socially engaged Zen Buddhism. The group is led by Rev. Dr. Harvey So Daiho Hilbert-roshi, an ordained Zen Priest in the Soto tradition. Daiho Hilbert-roshi also teaches via Daibutsuji Zen Temple Zen Center of Las Cruces
Dogen Sangha: The Dogen Sangha Buddhist Group is located in Bristol, England. Dogen Sangha is a worldwide group of Buddhist practitioners under the guidance of Gudo Wafu Nishijima Roshi who was a student of Master Kodo Sawaki. The Bristol group is led by Mike Luetchford. A collection of his talks is available here. His talks on the Shobogenzo are available here.
Egely Monastery: This is the website of Eido Shimano Dharma heir Denko Mortensen. Although mostly in Danish, there is some English content. The link will take you to the English section.
Engaged Zen Foundation: Not so much a Zen center but a Zen group committed to prisoner rights and social activism. From their site: "EZF is inexorably committed to the abolition of punitive incarceration in any form, the dismantling of the prison industrial complex, and the adoption of alternative, restorative, methods of dealing with what is colloquially known as "criminal justice."
Glasgow Zen Group: "The Glasgow Zen group was established in 1990 and practices within the Soto Zen tradition. We are an independent group of practitioners endeavouring to take a constructive part in the on going reception of Zen in the West."
Hsu Yun (Empty Cloud): There seems to be some confusion over the sites devoted to this great modern master. The Zen Buddhist Order of Hsu Yun is one site. There are extensive translations of his teachings here as well as poetry, dharma talks by the teachers of ZBOHY, canonical writings and prayers and photographs. A large site. A notice on the site says it has moved. The new site is labelled as Zen and the Martial Arts. It includes a link (hard to find) to the "old site".
There is a third site labelled ZBodhy, the Zen Buddhist Order of Hsu Yun. It has a disclaimer on the site: "The Zen Buddhist Order of Hsu Yun (ZBOHY) under the guidance of Rev. Chuan Zhi Shakya is represented on the www.hsuyun.org Internet website and is not associated with other groups or websites that might represent themselves as ZBOHY in the United States or other countries." Furthermore, there is a zen site, Exploring Chan, "from the friends and members of ZBodhy but whether this has anything to do with the "official" Hsu Yun group, I have no idea. Confusing, isn't it? In either case, all the sites have extensive collections of writings and talks and are worth visiting if you're interested in Hsu Yun.
Kwan Um School of Zen: this is the large site of the school of Korean Son Master Seung Sahn. Large collection of teachings here.
Lotus Zen Temple: located in Lincoln, Nebraska, this group is part of the Zen Buddhist Order of Hsu Yun. Has some basic information about Zen practice.
Montreal Zen Center: this is the late Albert Low's center. There are many books and articles available for exploration.
Mountains and Rivers Order of Zen Buddhism: main teacher is John Daido Loori, dharma heir of Hakuyu Taizan Maezumi Roshi.; located in Catskills Mountains, New York state. Has extensive section on Dharma teachings. You can also subscribe to their journal, Mountain Record: the Zen Practioners' Journal. Extensive recommended reading list here. Sounds from Zen Mountain: podcasts of teachings by teachers of Mountains & Rivers Order.
Daido Roshi died on October 9, 2009. The Mountain & Rivers Order abbot is now Geoffrey Shugen Arnold, Sensei.
Open Mind Zen: Zen Master here is Al Rapaport, who studied with Kozan Roshi, Maezumi Roshi, and Genpo Roshi. Has an excellent collection of MP3 Dharma talks.
Ordinary Mind Zendo: led by Barry Magid, dharma heir of Charlotte Joko Beck. This site has a number of dharma talks by Magid as well as talks on koans.
Prairie Zen Center: located in Champaign, Illinois, the resident teacher is Elihu Genmyo Smith, first Dharma Heir of Charlotte Joko Beck, co-founder of the Ordinary Mind Zen School. The site has a link to dharma talks and verses and gathas used by the center.
Rochester Zen Center: founded by Philip Kapleau-roshi, the site has articles and podcasts by the current abbot, Roshi Bodhin Kjolhede. You can find chants in MP3 here. You can also download the chant book.
Spirit Rock: this is not a Zen center but a vipassana center. However, I've added it here as it has a "Teacher Code of Conduct" on it's site, something I think is valuable. If you are in doubt about your teacher, ask yourself, Does he/she follow this code?
Sanmonji: a Zen center in New Mexico led by Mitra Bishop-sensei, a student of the late Philip Kapleau and Harada Shodo. Has an extensive list of teishos for sale on CD's but you can listen some for free here.
Throssel Hole Abbey: a Soto Zen Buddhist Abbey begun some 30 years ago by the late Rev. Master Jiyu-Kennett, the site has a number of dharma talks by the current abbot, Rev. Master Daishin Morgan, as well as by senior disciples.
Treeleaf Zendo: This is an online zendo, promoting zazen via the web. Certainly worth checking this out. Led by Jundo Cohen from Tsukuba, Japan, the teacher received Dharma Transmission from Master Gudo Wafu Nishijima, and is a member of the Soto Zen Buddhist Association and American Zen Teachers Association.
Upaya Institute and Zen Center: a very active center led by Joan Halifax Roshi who studied with Seung Sahn, received the Lamp Transmission from Thich Nhat Hanh, and was given Inka by Roshi Bernie Glassman. Includes writings by Roshi, podcasts, and newsletters. Site map here.
White Wind Zen Community: taught by Anzan Hoshin Roshi, based in Ottowa, in the Dogen tradition. Has good collection of teachings and translations (primarily but not exclusively, Dogen's writings).