The platform offers commission-free trades. Robinhood makes money from uninvested cash, a premium service, margin interest, and other ways.
Stash does offer $5 cash for signing up and depositing $5 in your account. You can take the $5 and buy free stocks with it.
Acorns is similar to Stash in that it is a micro-investing platform, and it is also an automated investment subscription service.
Webull will give you between $3 and $300 in one free stock just for opening a brokerage account with them.
M1 Finance has dozens of portfolios which M1 refers to as pies that are preset, or you can create the exact portfolio that you desire.