NGluon: A package to calculate one-loop multi-gluon amplitudes We present a computer library for the numerical evaluation of colour-ordered n-gluon amplitudes at one-loop order in pure Yang--Mills theory. The library uses the recently developed technique of generalised unitarity. Running in double precision the library yields reliable results for up to 14 gluons with only a small fraction of events requiring a re-evaluation using extended floating point arithmetic. We believe that the library presented here provides an important contribution to future LHC phenomenology. The program may also prove useful in cross checking results obtained by other methods. In addition, the code provides a sample implementation which may serve as a starting point for further developments.