As of Vim7, spell-checking is a built-in feature.

The spell checker will highlight incorrectly spelled words in red. Light blue highlights indicate words which are only found in another dictionary (such as "color" from en_us).

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Commands

Enable/Disable

Enable spellcheck

• set for all buffers, setlocal for the current one
• en_ca will use Canadian spelling.

:set spell spelllang=en_ca
:setlocal spell spelllang=en_ca

Or if you're not picky, the spelllang argument can be omitted. I'm not sure what the default is, but I figure it's probable en which uses all the English dictionaries together.

:set spell

Disable spellcheck:

:set nospell

Using Spell-Check

Move to the next misspelled word

]s

previous misspelled word

[s

Highlight a misspelled word. Show possible replacements. Press return with no input to go back.

z=

Accept the spelling of a word:

zg

Mark a word's spelling as wrong

zw