URLs are for humans - a browser concept
I agree with Andreas Bonini - URLs are for people. They should be editable, and sensible editing of a URL should not result in a 404. However, I think the idea of editing the URL to explore a site is very foreign and scary to the average user. It may seem straightforward to you and me, but for grandma this is nothing short of super advanced computer hacking. The solution?
Make the URL bar act as a series of links
As in this mockup.
On a page that lacks clear navigation, or one that lacks navigation entirely such as a direct image, I think this could be immensely helpful. Compare it to a clickable link and a plain text URL that you have to copy and paste into your address bar. It’s not hard, but it is a chore.
And if you do want to edit the URL? Click and hold for one second.
What do you think? Discuss on Hacker News
Update: HN user DHowett has kindly pointed out that similar functionality is available as extensions for Firefox here and here, and, with a somewhat different implementation, for Chrome.