Seems like a great idea to me as car parks can be so expensive. Especially useful if you are visiting a city you do not know very well.
You type in the city or attraction and the site tells you where you are able to park for free.
I used it to find free parking near a hospital as the car park is very expensive and it gave me an alternative!
Hope it's useful to someone.
They don't have all the cities on yet but do have this message:
We have extensive coverage in London and surrounding areas, Birmingham, Brighton, Reading, Luton; moderate coverage in Manchester, Oxford, Cambridge, Glasgow, Nottingham, etc. We also covered several smaller towns and many important toursist attractions. Please search to see if your town exists. We are adding new places frequently. All places, currently not available, will be added soon.http://www.freeparkingspace.co.uk/