A rainmaker is any person who brings clients, money, business or even intangible prestige to an organization based solely on his or her associations and contacts. Traditionally, the term "rainmaker" has been applied to members of the legal profession, like attorneys-turned-politicians who retired from public life (forcibly or otherwise) and went on to practice law at nationally recognized firms.
Over time, the term gained usage in many other industries and activities, including investment banking, political campaigning and public speaking. For example, former U.S. Vice President Al Gore's association with the environmental movement caused numerous venture capital firms to add Gore as a partner in hopes that his "rainmaking" abilities would help them raise hundreds of millions of dollars for alternative energy initiatives.
In everyday usage, the term "rainmaker" can apply to anyone – from the salesperson who always finishes first in billings to the marketing guru who consistently finds innovative ways to present a company's products. In the purest sense, a rainmaker is a difference-maker.
This question was answered by Justin Bynum.