I would suggest using simple htaccess method. Here's a quick tutorial: http://www.besthostratings.com/articles/password-protection.html..

Using .htaccess is very simple. You don't even have to create a login page, because the browser will popup a login dialog, in which you can fill out name and password.

This may not be quite what you're looking for. The system is in use in some Finnish advertising agencies. The designer post a pdf for the client to review, the client can then make comments to it online. The history of comments stays in the archives.

http://www.kommentoi.com/content.asp?lang=eng

I'm going to look into the suggestions above. The method I have been using is very simple. I just create a directory for the client and email them the address to the file...

www.domain.com/proofs/client/filename.pdf
or
www.domain.com/proofs/client/filename.jpg

It's not secure, but without the filename no one is going to find it. And for my work it wouldn't matter if they did.

There are dozens of way to handle this. The quickest is likely the htaccess method (if you are on an Apache server). Give each client a folder and password protect each folder.

The ones where a client logs in on the home page would be applications written in PHP or ASP.net or RoR or the like.

Thanks for the advice guys, some pretty intuitive suggestions which I'll take under consideration. I'm thinking I'll go with the htaccess method to display a knowledge of extra-curricular coding (I'm currently a student and our portfolios are graded by lecturers on a quarterly basis) so I appreciate the insight.