Although Bulgaria is already an EU member state, you can still find here fairly low-wage and yet well educated employees. Many western companies outsource their software routines to Bulgaria: development of stand alone application, web applications (php, .NET , java, ajax etc.), ERP. Usually the software companies choose to establish a separate business in Bulgaria. This means that the subsidiary company will be independent in its relations to any business partner, but yet subordinate via its capital owner - the mother company.
But not only big software companies make investment in Bulgaria. Outsourcing of software development involves freelancers too. Usually only a project manager is needed whose duty will be the distribution of the work among the freelance developers.
Because of the complicated employment/assignment relations, it is highly recommended that the outsourcing company consider registering a local company which will take care of the administrative work, regarding the development of the project, including taxes management, social security, office premises, business relations etc.