• robinhood is the biggest commission-free broker
• M1 https://www.m1finance.com/ Supports fractional shares, possibly the first among commission-free brokers.
• https://www.firstrade.com — commission-free
• TD Ameritrade might be commission-free
• IB says zero commission for U.S. customers, but am not sure.

See https://topratedfirms.com/account-charges/hidden/robinhood-hidden-fees.aspx

https://thecollegeinvestor.com/20432/buy-fractional-shares-invest/ also mentions

• Public — commission-free, supports fractional shares.

https://topratedfirms.com/account-charges/hidden/robinhood-hidden-fees.aspx praised Webull:

• Webull offers much better trading tools than Robinhood. Their flagship desktop system offers many professional-level features. Charting is much better than Robinhood’s app can provide.