Gardening hotline

Try the Gardenweb forums. There's a huge variety of them: regional, different types of gardening, specific plants, etc. The vegetable gardening and organic gardening forums are both quite active.

http://forums.gardenweb.com/forums/

The USDA site on gardening extension programs has a lot of helpful links.
http://www.csrees.usda.gov/Extension/
And your closest University or College will typically have an extension program that has courses as well as online resources.

You might want to check out this forum: http://forums.organicgardening.com/, and depending on where you live, there might be a location-specific option too. For example, after a quick search I found plant helplines for Cincinnati (http://www.civicgardencenter.org/helpline/) and Atlanta (http://atlantabotanicalgarden.org/plant-help/plant-hotline).