Rose petals and violets are often eaten, but I wouldn't eat them (as a previous poster mentioned) unless they were packaged for consumption or I grew 'em myself. They are so pretty when they are candied and used as a decoration for cakes and cupcakes.
Lavendar and some sage flowers are edible (some sages are NOT edible and can be psychotropic, though, so please be very very careful), but the flavor is very strong. I'd use 'em in small quantities in a salad that had some pretty strong flavors already for a dash of pretty. I've had some very tasty ice creams made w/ lavendar.
nasturtiums make a lovely infusion for vinegar.
Pick a generous bunch of unsprayed, unfertilized blossoms. Rinse them off and pack them in a jar with a wide mouth. Fill the jar with seasoned rice vinegar. Let them sit for about 2 weeks. Strain. Pack the jar again with as many blossoms as you can (unsprayed & fertilized; washed). Pour the previously strained vinegar back in and allow to infuse for another couple weeks. Strain again. Return to jar or a narrow necked bottle and, thereafter, store in a cool, dark place.
It has a lovely color and a slight peppery flavor. If you're tempted to put a blossom in for gift giving it will probably collapse before you can hand it over and your friend will probably be dismayed to see what happens to it within 34 hours. But you can put in a couple of unopened buds.
I have a large edible flower garden and use them for all kinds of cooking. For example, daylily buds are great stuffed and fried. Dianthus makes a beautiful jelly, nasturtium petals are beautiful when finely chopped and blended with softened butter and then used to top roasted salmon filets, and tulip petals can be cut into thin ribbons and used in tuna salad which can then be stuffed into whole tulips for a gorgeous presentation.
The following websites have some great information about using edible flowers. I can't emphasize this enough--Just be very careful because some flowers can be deadly if ingested. Children, older people and peopl with compromised immune systems should NEVER be given edible flowers to eat. And even if the flower falls into the edible category, be sure that is has been grow in organic conditions. Never eat flowers from florists or nurseries unless you are 100% aboslutely sure that they have been grwon under conditions which make them suitable for culinary use. You're better off growing them yourself.
As an aside, flowers used for garnish on restaurant served dishes may or may not be edible. Unless you are absolutely certain that they are, leave them on the plate. When preparing food with flowers as an ingredient or garnish, NEVER use flowers which are not edible or poisonous—even as a garnish. Always assume that your guests will eat whatever you put in front of them!
www.homecooking.about.com: Contains excellent information about edible and poisonous flowers as well as recipes.
www.whatscookingamerica.net: Has guidelines for culinary use of edible flowers along with helpful photos.
www.sagekitchen.com: Source for recipes and dried edible flowers, jellies and other goodies.
www.perennials.com: Has excellent information about over 70 edible plants for your garden and landscape.
I think there must be a list of do-not-eat flowers. But for any flowers you consume, be sure they are specifically labeled edible, or that you otherwise know for sure that pesticides/other chemicals have not been used on them.
Toss in salads, decorate cakes using either fresh flowers or candied ones. To candy: paint each petal with egg white, then dip in superfine sugar, shake off excess, and let air dry.