I was wondering if someone could write a program which would go through my entire list of bookmarks/favorites in my browser and 'mark' the sites that are no longer working.

I've just gone through one folder and am giving up on trying to clean it up myself.

Perhaps this would be OK. Above the description for this utility is a 'Bookmarks' link that leads to a page containing similar ones. Some free some not.

I have vague memories of Netscape including this feature in their browser perhaps 10 years ago, but I don't see it in recent versions. I used it and thought it was very helpful--saved a lot of time.