If you want to fill out your portfolio then just create some designs/redesigns of your own. Pick a niche and redesign it, for example online banking. As admirable as it is to do free design work, it's only work if you are getting paid and you will find that many times people will just take advantage of an offer like this.

Let's not forget the headaches of having to tweak to the client's happiness. Who wants to continue working on a site when you aren't getting paid to do so?

If anything offer a 1-2 page deal for $XXX. Better to have 2 paying clients than 10 free ones that you can't keep up with. Get in the habit of charging now so you don't feel weird about it later.