The Apple iPhone Appstore has crossed the half billion download mark. In just 6 months of its launch the Appstore has gathered an amazing 15000 apps (including several free apps). The Appstore had crossed the 200 million downloads mark in October, that means Appstore is witnessing an average 100 million app downloads per month since then.The Appstore is currently a significant advantage to the iPhone which is facing severe competition from Nokia, Samsung and HTC in recent times. No doubt Blackberry, Palm, Android etc are already in the process of getting a similar app market live (or have already launched).