Different implementations of Keccak, the standard SHA-3 and SHAKE functions, Ketje, Keyak and Kravatte exist. The first location to look for an implementation is the Keccak Code Package. For reference implementations and tools for cryptanalysis, we propose KeccakTools.
The Keccak Code Package (or KCP) gathers different free and open-source implementations of Keccak and closely related variants, such as the SHAKE extendable-output functions and SHA-3 hash functions, the cSHAKE, KMAC, ParallelHash and TupleHash functions, the Ketje and Keyak authenticated encryption schemes, the fast KangarooTwelve extendable-output function and the Kravatte pseudo-random function.
The KCP is organized in a way that one can easily switch between optimized implementations of the Keccak-p permutations for different platforms, without touching on the higher-level code.
Written in C and in assembly, the KCP also contains some standalone implementations in Python.
KeccakTools is a set of C++ classes that can help analyze Keccak and the related variants.