Weblio 英和・和英辞典 including Kenkyusha dictionaries.
Kotobank : search from 99 dictionaries and encyclopedia at once.
辞典横断検索 Metapedia : the site allows to search 551 reference books at once.
語源由来辞典 Japanese Etymological dictionary
古事類苑 Koji Ruien (1933) digitized by Internet Archive.
古事類苑全文データベース Koji Ruien full-text database About 10 % of 67,000 pages are digitized in this on-going project.
JAANUS: Online dictionary of Japanese Architecture and Art Historical Teminology Compiled by Dr. Mary Neighbour Parent
Some 20 reference books below are searchable at once, or individually.
Encyclopedia of Japan is a handy and reliable tool in English.
日本大百科全書（ニッポニカ) this encyclopedia search provides relevant web links as well.
Ministry of Finance Statistics 財務省統計資料
Last five years of figures pertaining to the government budget, government bonds, trade statistics, balance of payments, etc.
Ministry of Health, Labour and Welfare Statistics 厚生労働統計一覧
Ministry-issued statistical data in Excel format.
JETRO Trade and International Payments Statistics 日本の貿易・投資・国際収支統計
Both current and long-term (1958-2002) statistics are available in Excel format.
Public Opinion Polls / Surveys 内閣府 世論調査・アンケート
Full text of selected polls and surveys conducted by the government since 1977.
NHK Broadcasting Culture Research Institute, NHK, 放送文化研究所
Survey results provided under 研究・調査. Poll results available under 一般世論調査. Polls and surveys were conducted by the institute over the past few years. Brief English-language summaries of some reports are available under "Surveys and Research Results" at http://www.nhk.or.jp/bunken/index-e.html.
Data Archive, Social Science Japan SSJデータ・アーカイブ
The Social Science Japan Data Archive (SSJDA) collects, maintains, and provides access to quantitative data from social surveys for the academic community. Data is provided in Japanese only, but there is an English version of the website with information on how to deposit and access data. The archive is maintained by a unit within the Information Center for Social Research on Japan, Institute of Social Science at the University of Tokyo.