Ispell is an interactive spelling checker developed by GNU. Not only does it tell you that the word is spelled wrong, but it will suggest alternates. Ispell supports 'personal' dictionaries for each user. Some packages (elm, emacs) support ispell directly.
Since ispell requires a cursor-addressable terminal, it may not work properly from dialin if your terminal is not set properly.
ispell [ <filename> ]
Ispell can be invoked directly from some packages (see below).
M-x ispell-buffer. Elm supports using ispell before sending a message.
Other Topics of Interest