QuantConnect: This is a platform that allows you to design and trade algorithmic trading strategies in a web browser, with free financial data, cloud backtesting and capital. You can access various markets of the world, such as crypto exchanges, stocks, futures, options, forex, etc. You can also join a community of over 248K quants and collaborate with other developers1.
CodeCanyon: This is a marketplace where you can buy and sell scripts, plugins, code snippets, mobile app templates, and more. You can find various categories of code, such as PHP, HTML5, JavaScript, WordPress, etc. You can also browse ratings and reviews from other buyers and sellers2.
StockSharp: This is an open source project that provides tools for algorithmic trading and quantitative trading. You can use C# to create trading robots, indicators, strategies, connectors, etc. You can also connect to various data sources and brokers3.