The Japan Volunteers resource site has information and links for people who want to support earthquake / tsunami relief efforts in Japan:
http://japanvolunteers.wordpress.com/

Donation information for Japan-based giving (money, materials, volunteering) via Japan Volunteers:
http://japanvolunteers.wordpress.com/japan-donations/
http://japanvolunteers.wordpress.com/materials/
http://japanvolunteers.wordpress.com/volunteers/

Low-overhead Organizations
Many thanks to our members who have suggested giving to organizations with low overhead, as a greater amount of donations to these organizations go directly to the people in need. Below are some example low-overhead organizations. The Japan Volunteers site above also has links to many low-overhead organizations.

Second Harvest Japan (Based in Tokyo but with a disaster relief office in Sendai)
http://www.2hj.org/index.php/news/se..._and_supplies/

Hope International Development Agency-Japan (Goods, Volunteers (Nagoya), Donations)
http://www.hope.or.jp/en/suben/pr-2011-03.php

Habitat for Humanity Japan is planning to mobilize people living in Japan to help with clean-up (in the coming weeks)
Those interested in volunteering are asked to register their interest at info@habitatjp.org
Habitat for Humanity is also accepting donations through their website:
http://www.habitatjp.org/enblog/2011/03/donate_to_help_japan_earthquake_...