The modern mutual fund!
From Investopedia: “America's First Mutual Funds
The modern mutual fund that we know today first appeared in Boston in 1924 with the introduction of the Massachusetts Investors' Trust, which was the first mutual fund with an open-end capitalization, allowing the fund to continuously issue and redeem its shares. After only one year of existence, the fund's popularity was obvious. The fund's holdings grew from $50,000 to more than $390,000. The fund was also the first of its kind to go public in 1928. That same year saw the introduction of the Wellington Fund (now part of the Vanguard family of funds), which was the first mutual fund to include stocks and bonds, as opposed to direct merchant bank style of investments in business and trade. (See also: A Brief History of the Mutual Fund.)