Jisho (from the Japanese word 辞書, meaning dictionary) is the premiere online Japanese-English dictionary. It contains a huge number of features and integrations, which all together make it perhaps the most comprehensive Japanese-English dictionary on the internet — and best of all, it's totally free.
While Jisho does have all the features you might expect from a Japanese-to-English dictionary, such as the ability to search in English, Japanese, or Romaji, by drawing characters, radicals, or even a voice search, it's all the bonus features that make Jisho.org really special. For example, you can search full sentences, and Jisho will underline each item allowing you to search individually. There's also a number of modifiers, such as leading a search with "#name," which will show names that match your search, or "#common" to show only common words.
The entries themselves are similarly detailed. Vocabulary words give the meanings and readings of course, but also list an items JLTP level, whether or not it is commonly used, the WaniKani level, a button to play audio (which is also taken from WaniKani), collocations, and a "links" button, which will automatically take you to the listings for the kanji used or a sentence search for the item. On the page for kanji results, in addition to the meaning and the 音読み (on'yomi) and 訓読み (kun'yomi) readings, you're shown the number of strokes and the stroke order, the components of the kanji, compounds that use the kanji, and stats such as the frequency and what level a kanji is taught at.
While there's a lot of benefits for Japanese learners to use a monolingual Japanese dictionary, especially for beginner level learners, it's hard to beat Jisho. In particular, the features to help break down and understand sentences, as well as the wide range of information available make this an indispensable resource for Japanese learners.