These free resources are helpful for looking up Japanese words, learning about topics in Japanese studies, and finding books and free, open access texts in Japanese.
There are many websites, portals, and digital archives available with Japan-related resources. Some are in English, although many are only in Japanese. For a more comprehensive guide to these resources, check out the Japanese Digital Resources Guide.
There are a huge number of mobile apps available for both iOS and Android that help you learn and research Japanese. To learn about apps specific to Japan, Japanese, and Japanese news and culture, check out the Japanese Mobile Apps guide. This guide also includes helpful Twitter feeds related to Japan.
These are Japanese-Japanese dictionaries that provide not only word definitions, but also brief encyclopedia entries for people, events, places, and concepts. They are a good place to find the pronunciation of personal names and place names, as well as dates, if the terms have no entry in Wikipedia.
These are both free English-Japanese/Japanese-English dictionaries, with a slightly different focus. WWWJDIC is more academic and focused on traditional word definition, whereas Eijirō has a focus on contemporary usage and translation of whole phrases. Although both contain many example sentences, Eijirō is stronger in this aspect. Use Eijirō for writing emails, speeches, and essays in Japanese (especially using keigo); WWWJIDC may be more appropriate for defining less commonly used words or looking up kanji.
These reference sources are mostly in English and contain a wealth of information related to Japan. Japanese Wikipedia can be especially helpful for starting your research and looking up background information on a topic, as well as finding name pronunciation and dates. It's also good for upper-level language practice.