For targeted lists, I've had success with broad companies like Data.com, ZoomInfo and NetProspex, though my preference is Data.com. I've also been fortunate to find sources specific to some of the industries I've served - finance and government specifically. As for appending services, I've had limited use but have completed a few projects with DataFinder.
For List appending and sourcing new names. I have used Lead Space, Zoom Info, Social 123, Data.com and ReachForce. They vary across the board in quality and price and you really get what you pay for so it depends on your needs.