GitHub Copilot
AI pair programmer built into your editor.
Copilot was the tool that popularized AI code completion — suggesting whole lines or functions as you type, based on the surrounding code and comments — and now includes a chat interface for asking questions about a codebase directly in the editor.
It works as an extension inside existing editors like VS Code and JetBrains IDEs rather than as a standalone app, which keeps the workflow close to how developers already work.