For Vim/Neovim users, there is one built in that is pretty good, and once you've added frequent custom words to the dictionary it is great. You can turn it on with `:set spell` or off with `:set nospell`. Add custom words by pressing `zg` on the target word:

I have this in my vimrc file so it's on by default for certain file types:

    " Turn on spellcheck for certain filetypes and word completion.                                                                                                                                                                               
    " words can be added to the dict by pressing 'zg' with cursor on word.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
    autocmd Filetype markdown setlocal spell                                                                                                                                                                                                      
    autocmd Filetype gitcommit setlocal spell                                                                                                                                                                                                     
    set complete+=kspell                                                                                                                                                                                                                          
                                                                                                                                                                                                                                              
    " Don't highlight in red an underscore (_) in markdown                                                                                                                                                                                        
    " https://vi.stackexchange.com/q/18471/17441                                                                                                                                                                                                  
    autocmd Filetype markdown syn match markdownIgnore "\v\w_\w"

> custom additions

You can also add it directly when using it. Move the cursor to the word and (I forget the command...) can add it as a rare word, good word or bad word.