General

Mills Archive Trust: extensive collection of documentary and photographic records of mills, many of which can be viewed online. It also has a shop selling mill-related books and images.
The Templeman Library at the University of Kent: houses collections of mill photographs and documents
Windmill World: Information on many mills (both wind and water powered) in the UK and elsewhere
Mill Drawings: Computer-generated technical drawings of windmills and watermills across the UK and beyond
Crafty Tech: Computer-generated animated drawings of vintage mill and other machinery

Milling and baking

Traditional Cornmillers Guild (TCMG)
Campaign for Real Bread
Cope Seeds & Grain: Organic grain suppliers in Lincolnshire

See our Technical Advice page for information on Health & Safety
See our Planning and Conservation page for Bread and Flour Regulations

Water management and hydropower

Fish pass approval
Harnessing hydroelectric power
British Hydropower Association (BHA)

See also our Planning and Conservation page

Regional and international mill websites

Essex Industrial Archeology Group 
Hampshire Mills Group
Midland Wind and Water Mills Group
Norfolk Mills
North East
North West Mills Group
Suffolk Mills Group
Sussex Mills Group
Wessex Mills Group (Devon, Dorset, Somerset and Wiltshire)
Cymdeithas Melinau Cymru (Welsh Mills Society)
Mills and Millers of Ireland
The International Molinological Society (TIMS)
De Hollandsche Molen (The Dutch Mill Society)
FDMF (Federation des Moulins de France, the national mills group for France)