Chrome extension which shows furigana

NHK-Japan's national news services--easy to read furigana kanji and current news stories

Adds hiragana to Japanese language websites

Provided by the University of Chicago, Kanji Alive allows users to search for "kanji using a number of different criteria: textbook lesson number, onyomi, kunyomi, English kanji meaning(s), radical name, radical stroke number, radical English meaning(s) and kanji stroke number."
Also has support for Japanese textbooks including Genki and Tobira.

Jisho is a powerful Japanese-English dictionary. It lets you find words, kanji, example sentences and more quickly and easily.

Online dictionary. Especially useful for synonyms

Kotobank allows you to search across 77 dictionaries at once, covering variety of fields, including encyclopedia, languages, history, business and current issues. Provided by Asahi Shinbun,

Good for foundations of kanji including radicals. Has both free and (paid) premium sections

Smartphone app and web version. Useful for starting a language and forming language learning habits.

"Japanese" smartphone app
Look up kanji through text, drawing, or pasting. Useful dictionary of Japanese words, kanji, and radicals
Also has flashcard feature

Flashcard app--useful as computer or smartphone application. Tobira: Gateway to Advanced Japanese (Advanced textbook for Japanese Language) has premade Anki flashcards available for download (with purchase of the textbook)