NumPy
Foundational package for numerical Python — N-dimensional arrays, broadcasting, vectorized operations. Travis Oliphant, 2006. The base of the entire Python ML/data stack.
Merged Numeric (1995) and numarray (2003). Built on BLAS + LAPACK (Fortran libraries from the 70s) for linear algebra. NumPy 2.0 (June 2024) first breaking release in 17 years. Pandas, SciPy, scikit-learn, PyTorch (eager mode) all NumPy-compatible. CuPy is the NumPy API but runs on GPU. JAX is NumPy API + autograd + JIT. Travis Oliphant later founded Anaconda + Quansight.