Please check out a list of selected integrated apps here.

Easy Integration with Alpaca Stock Trading API for Developers
Alpaca is a modern platform for trading. Alpaca’s API is the interface for your trading algorithms, bots, or applications to communicate with Alpaca’s brokerage and other services.

TradingView has been one of the most used integrated apps among the users where TradingView users can buy and sell US stocks with commission-free directly from TradingView's web-based charting interface.

Alpaca + TradingView: Commission-free trading using TradingView charts
Alpaca is a modern platform for trading. Alpaca’s API is the interface for your trading algorithms, bots, or applications to communicate with Alpaca’s brokerage and other services.

Also, please see more examples and how to here.

Alpaca Works With - Documentation | Alpaca
Alpaca API lets you build and trade with real-time market data for free.

Commissions Disclosure: No minimums. Commission-Free trading means that there are no commission charges for Alpaca self-directed individual cash brokerage accounts that trade U.S. listed securities through an API. Relevant SEC and FINRA fees may apply.