This is merely an overview of popular Internet gardening merchants I buy from or hear a lot about. For more specific information on each merchant try our merchant reviews directory.
Spring Hill is a popular and famous mail order & online nursery. They offer the full range of plants, mostly flowers & perennials though. Many people do not like Spring Hill, complaining of plant size and quality. I think you get what you pay for and Spring Hill still offers some of the best deals around. When I spend so little for plants I'm not expecting much. The trick is to buy plants that you know will thrive in your area and spread rapidly, that way size doesn't matter, only price. Plus I've called them on their lifetime guarantee before with no problems.
Gardens Alive! offers primarily organic gardening tools, implements, and chemicals. I find their prices to be a little high, but some things are just hard to find locally. What is really nice is that their catalogues contain useful articles.
Gardener's Supply Company has a really nice selection of gardening equipment, gadgets, and outdoor decor. I find their prices to be high though. Some items you can find anywhere else, but many can be found at Lowes or Home Depot for significantly less.
Direct Gardening represents many small growers. They have a crazy selection and some excellent prices. The only issue is that shipping speed and service can be somewhat lacking since you really are dealing with multiple small companies instead of one large one.