pandas
Python library for data analysis — the DataFrame structure became the de-facto standard for tabular data. Wes McKinney, AQR Capital, 2008.
Open source. Sits on top of NumPy. pandas 2.0 (2023) introduced Arrow-backed DataFrames. Slow compared to modern alternatives: Polars (Rust), DuckDB (columnar SQL), Dask (distributed). But pandas is default in nearly every Python data science stack. Wes McKinney later drove Apache Arrow + founded Ursa Labs (now Voltron Data).