I usually add the "clear form" button to HTML forms by default. Is this button actually necessary or a holdover from another time? I have never gotten to the end of the form and thought "I screwed up, I need to reset this!".
The nice thing about the reset button is that it will repopulate all form elements with their original values, not simply set them to zero or blank. So if the form was generated by server with saved data, the user makes a bunch of changes, and then realizes not only that something is wrong but that they also have no clue what the original value was, reset is VERY handy.
Also it's nice for forms with lots of numerical data, like the timesheet page I'm working on right now. There are potentially 16 fields, all with generic, somewhat meaningless numbers. If the user figures out they were looking at the wrong schedule, it's nice to just nuke it back to what the server loaded.
Having said all of that, my page does NOT include a reset button, simply because I didn't want to leave open the exact opposite of awesome it presents, which is "and I'll just click this button to save...oh shit."
What I do instead is any field can be set to 0, but any non-valid data (non-numeric, less than 0, greater than 16) will revert back to the value last entered (which is stored via js). Doesn't offer the grand sweep, but it at least lowers the amount of possible data entry errors and keeps the user from losing data over a simple mistake.
456 has a great article and link on this topic, by the way.