These are all email newsletters and list-serves that I've signed up for and found to be particularly worthwhile:
The list-serve run by the American Community Gardening Association is a golden resource. I also just discovered that it has an archive online, so you can search just about any subject that anyone has asked or answered about community gardening in the last several years. Wow!
A more general forum, the Community Food Security Coalition list serve, is currently on fire with passionate people interested in local and sustainable food systems.
The folks over at City Farmer are also some of the most excellent and experienced gurus of urban agriculture. Somehow they always find the most interesting and ground-breaking news stories on the subject.
Do you have any favorites?