Vim's abbreviation feature also doubles as an on-the-fly spell checker. Simply create abbreviations for words you misspell often and Vim will fix them for you:

iabbrev mispell misspell
iabbrev funciton function
iabbrev functiton function
iabbrev fucntion function
iabbrev funtion function
iabbrev erturn return
iabbrev retunr return
iabbrev reutrn return
iabbrev reutn return
iabbrev queyr query
iabbrev htis this
iabbrev foreahc foreach
iabbrev forech foreach

For a list of English spelling corrections you can use immediately, check out autocorrect.vim.

CommentsEdit

Alternately:

set dict=/usr/dict/words
set spell

and turn completion on.