It must be terribly difficult to create an English grammar and style checker, because I've tried a few and have yet to be impressed. As a native English speaker and a person who writes and edits for a living, though, I'm probably a hard sell--and likely not even the target audience.
WhiteSmoke 2009 strives to be a comprehensive writing software package that offers grammar, punctuation, style, and spelling checkers, plus dictionary and thesaurus functions. The program even provides a link to WhiteSmoke's online English lessons, plus dozens of fill-in-the-blanks templates for business, legal, and personal uses (including employee applications, general power of attorney forms, various apology and condolence letters, and many more).
The file linked to this review runs a "how-to-use" demo as it downloads and installs the 7-day trial version. Be sure to watch the demo, as it is easy to misuse the program. For example, I tried entering text into the interface and clicking Apply to apply the changes--and crashed the program. You need to select text in another program and press F2 (or click a pop-up box) to import text into the WhiteSmoke interface. You then read through any changes and accept what you will, or edit your prose as needed, and then click Apply to make your changes take effect in the source document. WhiteSmoke documentation says the program works in "any application that features text entry." This was mostly true--I tried it in every text editor I've got, plus Microsoft Word, Firefox, and Internet Explorer, but I couldn't figure out how to get it to work with Excel.
In my tests WhiteSmoke missed spelling and grammatical errors (such as "com" for "come," "your" for "you're"); it consistently missed inappropriate or missing plurals; and it suggested "could" for "was able to" when the latter was fine in context. In one case it turned an unwieldy run-on sentence into three fragments (though it did complain about sentence structure). Its pop-up synonym suggestions could be useful, however, and it did offer some helpful suggestions for style.
Overall, WhiteSmoke could be useful for non-native speakers of English, or people with a shaky command of grammar. I wouldn't depend on the spelling checker function--I'd use Word's build-in feature--and I'd accept some of the grammar suggestions with caution, but on the whole its suggestions seemed reasonable. Its generous collection of templates could be useful for people who are not confident in their writing abilities. At any rate, the seven-day trial is free; you should know within an hour or so whether this program is right for you.