Below is a list of flowers from the game, where to obtain them and how much they sell for.
Flowers (along with forage & Tree Sap) are one category of the items scored in the grange display at the Stardew Valley Fair.
Flowers (aside from Sunflowers) that are grown as crops will affect the type of Honey produced by nearby Bee Hives.
Sunflowers can be processed in the Oil Maker to produce Oil.
Notes
Daffodil and Dandelion are classified by type in game as forage rather than flower as would normally be assumed. However they are treated as flowers in dialogue with NPCs and included in the same category for the grange display and so are included here for completeness.