Everyone can make a spelling mistake once in awhile but sometimes its just typos. I use my browser's built-in spell checker - it checks my spelling while I type so its instant. I wish it could check grammar too.
I do not generally need the spell checker as I am an excellent speller and an above-average writer (as evidenced by my academic writing scores).
One problem in any internet forum is the variety of people we have here. As other posters have mentioned, there are different spellings and even different rules of grammar and syntax depending on writing style. I generally use MLA (Modern Language Association) format, but I know other styles such as Chicago and Turabian that people use depending on their academic discipline. All are valid and similar so, what can look like a mistake may be a different format.
I'm pretty good at recognizing when a word doesn't look right, and there is always the red squiggly line that shows up when there is a misspelling. The grammar is the difficult part. Google Docs actually is great for going through grammar for writings.