I long used vimspell plug-in for vim, but now, vim 7 include a spell checker !

To enable:

:setlocal spell spelllang=en_us

However, I’m French and by default French dictionaries are not installed on debian, so

mkdir -p /tmp/vimspell
cd /tmp/vimspell
wget http://ftp.vim.org/pub/vim/runtime/spell/fr.utf-8.sug
wget http://ftp.vim.org/pub/vim/runtime/spell/fr.utf-8.spl
wget http://ftp.vim.org/pub/vim/runtime/spell/fr.latin1.sug
wget http://ftp.vim.org/pub/vim/runtime/spell/fr.latin1.spl
sudo cp * /usr/share/vim/vim71/spell

and that’s it :setlocal spell spelllang=fr works!