kantango
This is a resource more than a lesson site. The main section is a search engine/dictionary. If you sign up (it's free) you can create word lists and access other people's by clicking on 'provider'. I recommend "Brookes" which is a university and has plenty of lists from the Basic Kanji series and Minna no Nihongo plus some poetry for interest. I mostly use it as my go-to quick stop for looking up kanji. It's about 95% accurate, but occasionally i need to double-check for nuance.
GOOD POINTS:
You can input romaji if you don't know the hiragana or kanji
It holds the majority of common Japanese names, which is useful, although it does give you EVERY name option, so in cases where you've no hiragana and no idea how it's pronounced it can come up with 20 options.
You can create flash cards and custom quizzes
You can access other people's word lists and quizzes.
No ads; it's clean and easy to navigate
BAD POINTS:
You cannot look up the kanji if you only know the english word.
You cannot search by radical
Sometimes the definitions are a bit abstract.
It can be time-consuming to make lists.
Not a great teaching tool, but a good support for learners with other materials.