The always-excellent Denshi Jisho (jisho.org). Searches all kinds of terms and provides helpful references e.g. conjugation. This bookmarklet will search jisho.org for your current selection.
Weblio E-J Dictionary
Weblio's English-Japanese Dictionary offers a wider range of definitions including more obscure ones from various dictionaries and often has better usage cases. The website is, however, primarily in Japanese.
Unihan Unicode Character Database
When all else fails and you can't decode the Kanji/Character. Also contains useful information such as Unicode codepoints.