This extension by firefox makes developers and programmers life so much easier. They can check for their HTML code and debug it there and then.
Since I keep working with programmers all the time, I have to admit this is one of the most useful firefox extension in a long time. I got hold of it a little later in the day though (It was released in February 2006) but then as long as it still works for me :-)
Simply click the following link to download it and get going HTML Validator