yfinance : Stock data: history, intraday quotes, fundamentals. Plus FX, crypto, and options. Uses Yahoo Finance, so all data from there is available through yfinance.

pandas-datareader : Used to be part of pandas, now a separate project. Data on stocks, currencies, economic indicators, Fama-French factors, and much more.

IBApi : Official Interactive Brokers API with access to all their data. Replaced IBPy.

Alpha Vantage : Free API with real quotes and popular financial indicators. JSON or CSV format.

Nasdaq Data Link (formerly Quandl) : Millions of financial and economic datasets from hundreds of sources directly in Python.

Twelve Data : Access to 100,000+ tickers for stocks, forex, indices, and fundamental data worldwide.

Polygon.io : Real-time and historical data on stocks, currencies, and cryptocurrencies.

Tradier : Python libraries for working with the Tradier API.

alpaca-py : Anything you want: from streaming market data to developing your own investment apps.

Finnhub : Real-time REST API and websockets for stocks, currencies, and crypto.

marketstack : Intraday and historical data for 30+ years, 170,000+ tickers.

Tiingo : API with end-of-day quotes. Focus on reliability, transparency, and completeness.