Enterpreneurs: Successful business founders are the bulk of billionaires. (Though unsuccessful ones are often poor as dirt)
Note that many mid-size business founders can make $500k+ per year. Even a plumber who starts his own business and has 10–20 guys under him could be making a TON of money.
Lawyers: Successful lawyers who make partners at large firms can make millions
Business People: Successful business consultants and leaders who work in the C-Suite or Partner level at large firms and companies often make over a million dollars a year
Investment Bankers: These professionals help companies raise money and manage large volumes of investments. If an investment does well, they can earn a great deal in bonuses and commissions (sometimes into the millions)
Doctors: Doctors, especially in specialized industries, can oftentimes make a half million a year or more. For those that found their own practices in niche industries (ex: plastic surgery) the earnings can be even higher.
Engineers: Top engineers typically make $150k/year or more and the very top ones at major tech firms can make well over $500k/year
Public Figures: Public figures such as athletes, musicians, and politicians can sometimes make tens of millions of dollars, though oftentimes the income is sporadic and unreliable and oftentimes they live extravagant lifestyles that leave them broke