These perennial purple lobelia plants (Lobelia x speciosa 'Grape Knee Hi') are similar to the better known cardinal flowers (Lobelia cardinalis), except for their color, cardinal flowers being red.
There's also an annual lobelia, widely sold at garden centers. Despite sharing the name, "lobelia," it's easy to distinguish the perennial flower from the annual: the former has an upright growth habit, while the latter trails along the ground.