Tidy is a great little tool that checks the HTML or XHTML code you have written, and fixes any errors you have made, and also tidies up and ‘prettifies’ the code. It has a command-line interface, with myriad options, so it can get a bit bewildering (for me at least)
That’s where Tidy UI comes in. It’s a Windows front-end for Tidy that turns it into a text editor, with the tidying and correcting features in easy reach.
Both are free, and very small (84kb and 370kb respectively). I use these tools most of the time for editing the site code.