I've been using MS Word for grammar checking long documents - but that's all I use it for so I copy/paste and rarely bother to save the file. The last file that I proofed was three pages of text; Word found a hundred typos, spelling errors, and grammatical errors; I found a hundred MORE plus a hundred phrases that should be reworded.
I read and study a lot on the Internet and I rarely continue reading past a third error - even if they are all typos. Typos are fine in forums, chat, and personal emails - but typos and poor grammar are not acceptable on commercial web pages. I tend to disregard what I had read if I leave a page that was written by someone who isn't educated.
If you want your web site to be professional, you need to hire a professional copywriter. First, check it yourself with a regular grammar-checking machine. Then read it yourself and ask peers to read it also. When you are "sure" that it is perfect, pay a PROFESSIONAL copywriter to redline it and ASK in person (or on the phone) why they made their changes - so you can learn from your mistakes.
If it is a commercial web site, you should consider paying a professional marketing consultant to check it because copywriters are typically better at technicalities than they are at sales. I do well with both. | 
