When I build sites for clients, I like for them to see the progress as we go along. Instead of using their url, I set up a temporary site for them to visit.
There are a bunch of free hosting sites out there, but I have often found that they tend to be quite buggy and annoying. Zymic is a great one. I can add multiple sites to my account and they always connect fast and easy. Only problem with free hosting is that it has limited space, so if you happen to have a big image file on your site, chances are your free host won't be able to carry it.
Here is an example of zymic at work - Frances Chewning's Temporary website.