A tuner card to receive local broadcast channels and display them on your computer.
Hulu, Fancast Joost, et al provide an aggregation point for content from multiple networks and studios and serve it up through their site.
I think TVersity does this as well but you have to purchase their software application to do this.
I suspect there are media content aggregations sites that serve up material from around the globe thereby "picking up stations from around the world".