Blunt Object has a post up that's awfully good. RTWT. However, in that post, he references a Bryan Caplan post 3 times, meanwhile saying nasty things about people who don't click through. Rather than being properly circumspect like Blunt Object...I'm going to just come out and say it:
Open Borders is the single political decision that would most benefit humankind of all feasible political actions. Full F'in Stop.
Average estimate of global GDP increase from full free trade:
~1.5% (8 estimates)
Average estimate of global GDP increase from open borders:
108% (4 estimates).