The British Newspaper Archive is a partnership between the British Library and brightsolid online publishing to digitise up to 40 million newspaper pages from the British Library's vast collection over the next 10 years.
http://www.britishnewspaperarchive.co.uk/This is not a free site but for those of us who do not live in the city, it's probably cheaper than a visit to the archive! Registration is free and I think searches are free but have not yet had chance to check the site out fully.
Edit: I have managed to find some costing info....
A page from a newspaper published over 107 years ago (B&W) or scanned from microfilm (B&W) will cost 5 credits.
A page from a newspaper publlished over 107 years ago (colour) will cost 10 credits.
A page from a newspaper publlished within the last 107 years will cost 15 credits12 Month Subscription Price: £79.95 GBP
30 Day Package Credits: 3000 Price: £29.95 GBP
2 Day Package Credits: 500 Price: £6.95 GBP
It will be busy. The site only launched this morning and BBC Breakfast have a feature on it so, if past high profile launches are anything to go by, the site will crash