A great number of flowers are considered edible, but that doesn't always mean you should do so. The warning lies in how they were grown. Unless you grow them yourself or they are purchased from someone who specifically grows flowers that are meant to be consumed, you do not know if they have been treated with pesticides or other harmful chemicals. That said, Edible Flowers, a publication from North Carolina State University, has a very complete list along with instructions and warnings.
Here is a partial listing of edible flowers:
Borage, Calendula, Nasturtium, Chicory, Hollyhock, Hibiscus, Daylily, English Daisy, Johnny Jump-Up, Pansy, Signet Marigold, Rose, Snapdragon, Squash
