Better Writing

My students can tell anyone that I have an obsession with good writing. Most of my students could benefit from devoting more attention to the quality of their writing. The spell check programme that is built into Word and other similar programmes is a help, although sometimes things slip by -- principal and principle, lead and led, their and there - and so on. Word also has a grammar programme which is helpful. It catches obvious mistakes and at the very least provokes reflection about the clarity of sentences.
Perhaps readers of the blog know of better programmes? I have come across something on the internet, but I have not used it, called Whitesmoke. The price seems reasonable enough, given the benefits. Do any readers have experience with this? Are there competitive programmes?
Last year, one of my students invested a considerable amount of money getting a thesis reviewed by a firm located on the internet that promised to revise and correct. Finally, deadlines were not met and the quality of the work was questionable. Do any readers have experience with such firms? Recommendations of both good and bad services would be very much appreciated.

Finally, I advise doctoral students to acquire their own library of reference books devoted to good writing. They don't have to be as insane as I am, and purchase the twenty-volume Oxford English Dictionary. But it is necessary to have a good collection of dictionaries, style manuals, and so on. Some of these are available on CDs and can be downloaded onto a laptop. I have copies of the Oxford Shorter and the Oxford Thesaurus, as well as the Petit Robert for French, and use one or more of them virtually every day. The thesaurus has a very handy feature where you need only highlight a word in a text and immediately a menu pops up with the alternatives. I know that there is also a thesaurus in Word, but I don't think it compares with the Oxford version. By the way, the Oxford thesaurus can be purchased for about 10 pounds, if I recall correctly.