Seedy Sunday - seed swap events in southern England. This site also helps you run your own Seed Swap event. NEW
Charles Dowding - Organic gardening, the natural no dig way. NEW
The Plant Exchange - online plant exchange scheme. The downside is you have to register with a small annual fee.
Organic Gardening - A guide to organic gardening - information on plants, soil preparation, fertilizing, mulching and more!. 
Eartheasy - the site has ideas and information for environmentally sustainable living, their Grow section is a good source of articles for organic gardening. 
You Grow Girl - a blog with useful articles, ideas and tips.
Garden
Swap Shop - buy or swap plants online
Safe Gardening - garden safety tips and advice from experts.