Aspell is a "Free" and "Open Source" spell checker I'd recommend for those who are using LaTeX.
It goes through your text file and suggests possible replacements for a misspelled word. What you have to do is just decide whether that word shoud be replaced or not. You can simply use only this command:
aspell -c <file>
The option -c means to check a file and <file> is any text file except MS Word. There is a version for Windows as well.