I don't have time right now to read the whole thread over before replying so I apologize if some or all of these have already been mentioned, but here are my favorite edible plants and flowers.
All members of the viola genus produce edible flowers and I believe foliage as well. Violets, pansies, and johnnie-jump-ups are all edible and great for candying with sugar.
Nasturtiums I'm sure have been mentioned but I just have to bring them up again. They're one of my very favorites.
Roses and rose hips! Rose hips are very good for you and can be used for tea or jelly or syrup.
Opuntia cacti! The young pads and fruit are both edible and very good for you, high in calcium. If you keep bearded dragons, also, the young pads and fruit are excellent staple foods for them, too.
Miner's lettuce! Though this is usually something I see growing wild rather than in people's yards I strongly recommend it if you can find seeds or something. It's healthy and tasty too! Has a great texture.
Hibiscus! The flowers and, I believe, the foliage are both edible. I know for sure that the flowers are.
Catmint! It works on humans, too, not just kitties. It makes a really excellent tea and the fresh leaves can also be used as a spice.