I'm a user of httrack. It is a good software to mirror a web
site.
But I think it would be better if have this ability. I call it
fast-update.
When I want to update a web site mirrored in my hard-disk by
httrack,
htrrack access recursively the web site on internet. But
there are few files changed in that site. For example, I
mirror a site by:
httrack www.somesite.com/update.html
The file update.html have links point to a.html, b.html,
c.html, ...
respectively.
And a.html link to a-1.html, a-2. html, b.html link to
b-1.html, b-2.html,
c.html link to c-1.html, c-2.html, ...
Now, the site add a file a-10.html and add a link in a.html
to point to
this file.
Other files (b.html, c.html ...) have no change.
I want httrack action like this.
Firstly, it find the file update.html was changed and
download it.
Sencondly, httrack access all links in update.html and find
a.html was
changed and
b.html, c.html, ..., have no change. So httrack download
a.html only.
Thirdly and most importantly, httrack find files to update
in a.html
only, and ignore the links in b.html, c.html, ...., because
b.html and  c.html have no change. 
If httrack runs like the way discribed above, it will be faster.