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.

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, liturgy and chants used by the center, as well as photos and other materials.

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.

Glasgow Zen Group: Soto group based in Glasgow. Has a pretty good section on Dogen here. Also has lineage charts and links to temples.

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 and www.xuyun.org Internet websites and is not associated with other groups or websites that might represent themselves as ZBOHY in the United States or other countries." 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 Meditation Center: located in Seward, Nebraska, this group is descendent from Hsu Yun. Has some basic information about Zen practice.

Montreal Zen Center: the link will take you to the page of teishos and writings by Albert Low, the teacher. There's a link on the left which will reveal more teishos by year. An extensive collection.

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. Also offers selections online from 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.

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.

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?

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).