Denshi Jisho is an easy-to-use and powerful online Japanese dictionary. It lets you find words, kanji and example sentences by searching in many ways.
The dictionaries are also interlinked so that you can check what the kanji in a word mean individually or what context a word can be used in. You can also look up kanji by the parts they contain.
The data is publicly available and comes from the excellent WWWJDIC project.
Denshi Jisho uses Open Search so you can use the word search from your browser's search box. For example, to use this in Firefox, choose "Add Denshi Jisho …" from the menu in the search box.