NCSA httpd 1.0a4 has some remarkable changes. There are too many to detail
here, and the online docs are already written. If you don't have network
access, a .tar file of the online documentation is now being provided with
new releases of httpd.
Quickly, though, features added include user authentication, an improved
image mapping script with circles and polygons, and POST scripts. There are
many other changes, improvements, and bug fixes.
The online documentation at http://hoohoo.ncsa.uiuc.edu/ has been vastly
improved, even if you are upgrading I would suggest reading it all the way
through. It was difficult to get a clear grasp of what was happening in the
old docs, the new docs may prove enlightening.
If you are running httpd 1.0a3 and its ilk, especially if you are using
the access control features, I'd strongly suggest upgrading.
If you are using a version of httpd earlier than that, PLEASE upgrade,
since those versions of httpd have harmful bugs that confuse HTTP/1.0