Vim's spell checking feature is pretty great. I use it as a default for
.rdoc files on my machine.
Sometimes it incorrectly marks technical words like 'ExUnit' as bad (misspelled). To fix this, I type
zg over the word in normal mode, which adds it to a list of white-listed words that are no longer considered bad.
The opposite of that command is
zw, which comments out the line in the dictionary file. Run
:runtime spell/cleanadd.vim to clean up those comments.