i have a big html file where 1 single div tag is missing, don't know where. Is there any easy way to guess it?
PSPad highlights the start and end points of divs. Personally, I'd just sit down with a cup of coffee and work my way through the code, indenting each div as I go. The HTML Validator plugin for FF might help you find out where the unclosed div starts, but you're probably using that already
DreamWeaver also got a function that points out where the missing div should be. But I agree with Kerosene, you should rather look through your code manually. You will learn more from that than just using some kind a validator.
Post here the address or the html code and we will try to help you. A good way to see from where is missing a div tag si to validate your document : http://validator.w3.org