Do you ever have one of those days where you don't function properly? For me, it's working in front of my computer for five hours straight and my brain starts to go numb. It's hard to think and see writing errors. When I get those midday "brain farts" reviewing my work, I turn to COMMON ERRORS IN ENGLISH, a free resource at http://www.wsu.edu/~brians/errors/errors.html#errors. Run by Paul Brians, Professor of English at Washington State University, it lists hundreds of common and unusual grammar errors. His website has saved me from misusing words plenty of times.

Prof. Paul Brians' website is based on his book, "Common Errors in English Usage: The Book." Buy a copy at http://www.wsu.edu/~brians/errors/book.html