This perennial site is awesome. It allows you to search by a variety of characteristics including zone, color, blooming time, sun exposure, name and more. This is the site of Heritage Perennials, known for their distinctive blue pot and the coined term "The best perennials come out of the Blue."
One of our primary suppliers of our quality trees, shrubs and fruit. They offer an outstanding product and their web page is a tremendous resource.
This is very user friendly, offering information about one of the top annuals available with oodles of pictures and information. Try their container recipes, a sure success.
Ball Horticultural offers lots of neat ideas and tips for containers and landscape.