OmniThreadLibrary is a powerful threading library for Delphi. OmniThreadLibrary's main "selling" points (besides the prices, of course ;) are power, simplicity, and openess. With just few lines of code, you can set up multiple threads, send messages between them, process Windows messages and more. OmniThreadLibrary doesn't limit you in any way - if it is not powerfull enough for you, you can ignore any part of its "smartness" and replace it with your own code.

There are quite a few Delphi open source projects on GitHub that can really save you some time. I searched for Delphi on GitHub and then sorted by the Most Starred projects and came up with this Top 10 list. There is some really good stuff here including Awesome Delphi (which is another top list), Delphi IDE color theming projects, an MVC web framework, a REST API client, Open Source Computer Vision Library headers for Delphi, a really powerful custom TreeView component, a unit testing framework, a component for embedding Chrome in your app, and finally the fantastic OmniThreadLibrary.