British Library – Lots of free resources and good links

COPAC – A merged online catalogue of approximately 90 libraries in the UK and Ireland. Terrific for finding where sources are held

Khan Academy – Not a library but a learning resource dedicated to providing a free education to anyone, anywhere. Excellent videos and subject chapters

Library of Congress – Lots of free content and good links

National Library of Scotland – Lots of free content and good links

Saulcat – Catalogue of the University of St Andrews Library

Worldcat – Online catalogue of 72,000 libraries in 170 countries and territories. Great for finding out where sources are held