Recycle Now
Designed to raise awareness of recycling, this site has lots of advice and handy tips on composting (and recycling).

The Composting Association
Organisation promoting good practice in composting and the use of composted materials.

Henry Doubleday Research Association
Organisation dedicated to researching and promoting organic gardening, farming and food.

Master Composter
Provides information on how to compost, the benefits of composting, what to compost, and many other topics related to home composting.

Royal Horticultural Society
Lots of useful gardening information.

Soil Association
For useful information on gardening.