More than 500,000 eBooks can be downloaded free of charge from the World eBook Fair during October (membership is normally $8.95 per year).
The collection is a real mixed bag. It includes many titles readily available elsewhere (such as the Project Gutenberg collection), and it includes many long, dry government documents (such as WMD reports).
But there are some real gems too. My favourite section is Renascence Editions – an online repository of works printed in English between the years 1477 and 1799. For example, you can read Queen Elizabeth the First’s “A Proclamation agaynst the maintenaunce of Pirates” from 1569.
Probably the best place to start is with the World eBook Fair’s list of collections.