Denver-based SpotXchange, the operator of an online advertising marketplace for video ads, said today that it has racked up more than 50 publishers as users of its new First Call service. The service, launched in February, allows online publishers to set prices for advertising on their premium video inventory. Financial details were not announced by SpotXchange, but the firm did quote such sites as Metacafe, HealthGuru, and others are participating in the service. SpotXchange's service works by allowing publishers to set pricing floors on their inventory. SpotXchange says its network reaches more than 100 million unique visitors each month.