False Shamrock

Oxalis triangularis, commonly called False Shamrock, is a species of edible perennial plant in the Oxalidaceae family. It is endemic to Brazil. This woodsorrel is typically grown as a houseplant but can be grown outside in zones 8a-11, preferably in light shade.
The subspecies O. triangularis subsp. papilionacea, in particular its cultivar 'Triangularis', is highly popular as a pot plant and known as Love Plant, Purple Shamrock, or (like many relatives) "sourgrass.

http://en.wikipedia.org/wiki/Oxalis_triangularis