If you are fanatical about having the most comprehensive listing of companies available, then Hoover's may be for you. It consists of what is probably the largest datatabase of business in the world, a mind-boggling 65 million+ companies and 85 million+ individuals within those companies. Hoovers is a standard fixture at many professional sales organizations and the premium version lets you create lists and have access to more information that the free version.
BoomTown is missing a lower-cost option for smaller agencies and individual agents. Even though their lead generation marketing efforts and CRM combine the best use of communication and technology to produce a great product that stands out among real estate lead generation companies, BoldLeads or Real Geeks might be a better choice if budget is an issue.