I am a history teacher in training, and these are the sites I am finding useful.
British Library
"Projects Area" is the best part of the website. Teachers' Notes section as well as student worksheets. Clear, well-presented sources accompanied by contextual knowledge, from pictures to sound files from the speeches of Holocaust survivors (accompanied by notemaking frames). Very clear and straight forward, well linked with the National Curriculum. More teacher friendly than student friendly - not one kids could easily used unguided. Not as vast as the Public Records Office site, but what's there is very good.
Public Records Office
Masses of information, particularly in the Learning Curve section. Treasure trove sources - transcripts/scans of official documents relating to the topics - many with related tasks and links to the National Curriculum. Each section has a guide for teachers. Specialists write the sections - for example, exemplary teachers, people who work for exam boards &c. "Build Your Own Cartoon" exercise, part of the "Focus On.." section dealing with sources, allows pupils to piece together puzzles depicting political cartoons.
History World
Aimed at teachers, particularly useful for background reading. Some activities for children - timelines &c. - but would need simplifying. Organised around National Curriculum topics. Links to KS2 and KS3 at the bottom of each page. Site created by Bamber Gascoyne. Links to source providers - for example, information on the illustrations, with external links to similar pictures. "Tours Through Time" provide contextual links - eg, you can see what was going on around the world while the Civil War was happening in Britain. Biggest history website in the world - lots of institutions have linked into it.
institutions have linked to it.
Spartacus
Lots of information with formidable cross-referencing. Difficult to navigate and doesn't advertise its learning activities very well, which is just as well because they're not very good "Fill in the Blanks" and so on. Very dry - not very many sources, cartoons, moving parts moving parts &c. Good resources on curriculum related things, but you need to know what you're looking for and be prepared to spend some time digging.
School History
Lesson plans, resources, online activities and lots of useful links, all from the guru of history through ICT, Andrew Field.
Active History
The Wolverhampton Grammar School's history site. Lots of activities for children to do - challenging games etc with good learning outcomes. Easy to link into lessons - activities can be completed and then printed out for exercise books. Home of the "Interview With..." game - currently featuring Henry VIII and Hitler.
Simon Schama's History of Britain The BBC website to accompany the series.