Wordd - Quickly look up vocabulary, abbreviations, acronyms, slang
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Eunoia - Words that don't translate
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Yak Tack - remember. words.
Words For That - the reverse-Urban Dictionary of the internet