University of Illinois
A good publication on a variety of topics.

Better Homes and Gardens
Good gardening articles and a good searchable plant database.

Lowes
Good how-to guides for garden projects.

HGTV Gardening
Lots of information and how-to tips on just about everything for your garden.

Cornell University
A nifty flower search by criteria tool.

BBC Gardening
Projects you can do with children in the garden.

Perennials.com
Use to search perennials by criteria.

National Garden Association
A useful garden calculator to determine how much fertilizer or mulch to buy.

Everyrose.com
A rose reference database.