Arb is a C library for arbitrary-precision floating-point ball arithmetic, developed by Fredrik Johansson (firstname.lastname@example.org). It supports real and complex numbers, polynomials, power series, matrices, and evaluation of many transcendental functions. All is done with automatic, rigorous error bounds.
The git repository is https://github.com/fredrik-johansson/arb/
The documentation website is http://fredrikj.net/arb/
A PDF version of this documentation is available.