So pick something that requires very little capital, like saving the 16000 children who die every day from starvation-related problems in developing countries.

"I lack capital" is a very silly excuse given that the HN community are among the richest people in the world... by definition, if you have the luxury of even thinking about moving to Mountain View and starting some web site, you are among the richest people in the world. A lot of people wake up in the morning and their first task is to figure out how they are going to eat today.