Zeldman wrote about redesigning web sites for companies that muck it up at no charge. I think this is an interesting topic, related to the topic of redesigning your own old sites to meet today’s standards. I am all for redesigning poorly built sites.
The next benefit comes when a company realizes their site is crap and they rebuild it, maybe they hire you, maybe they don’t, but if it gets fixed then many people will be better off. If you are the catalyst for a positive change on the Web then you’ve done something worthwhile, free or not.
Finally, for those who are looking to prove themselves but lack the high profile project to do it you can show what you would do for potential clients or employers if given half a chance.
There really is no downside, as long as you are careful to acknowledge any relevant copyrights when you show your work you should be golden. So go wild and have fun showing the big boys of the web the right way to do things.