Furigana inserter add-ons are nice for Japanese learners who know vocabulary based on prounciation but struggle with matching the reading with the kanji. These add-ons, on most websites, are able to insert the furigana reading of kanji right above the kanji itself!

For Chrome users:

https://chrome.google.com/webstore/detail/ipa-furigana/jnnbgnfnncobhklficfkdnclohaklifi?hl=en

For Firefox users:

https://addons.mozilla.org/en-US/firefox/addon/furigana-inserter/

For Android users:

https://play.google.com/store/apps/details?id=com.furiganabrowser