Within the US, you can use someone like TriNet to handle employees in all 50 states. Companies like http://balsamiq.com/company/ are all over the world. From what I understand, people often get paid as contractors and have to deal with local tax laws on their own. If you end up hiring people as employees in all the countries, I think it gets a bit tricky. I'd love to know if there are other good ways to do this.