It's called Cistus or Rock Rose. Something I didn't know before looking this up, but it seems like all the different colors of this flower are actually called different species, with their own specific name, and not just varieties or cultivars of one species. The one in your picture looks like Cistus ladanifer, aka "Gum Rockrose".
This is a very interesting plant, used medicinally and in perfumes. And WOW, get this!: According to Wikipedia, "Various Cistus species are known to emit volatile oils, rendering the plants flammable. Some sources state that under dry, hot conditions these species may be capable of self-ignition." So, be careful out there.