How can I clean the glass door on my fireplace?
This is going to sound bogus but I just got this tip the other day from a friend and it worked great on my woodstove.  Take a wet paper towel and dip it into some cool ashes from inside the fireplace, just enough to get a thin coating of ashes on the wet towel.  Then scrub the glass with the wet towel.  Within a few seconds you'll see even the toughest creosote start to disappear.  I usually finish with a dry paper towel just to get it really sparkling.  I don't know how or why this works, but I suppose it's probably some combination of abrasion and a dissolving action.  It really is amazing.

Just FYI, if your glass is totally covered in creosote, it probably means your fireplace isn't burning hot enough, and/or you're burning too much sappy wood like pine, and/or you're burning wood that's not properly seasoned.  The creosote on your glass is also being deposited on the inside of your chimney, which is bad.  Pine can be burned but needs to be well seasoned and ideally should be mixed with seasoned hardwood.