MXNet provides a set of pre-trained neural networks in its Models Zoo. Model symbol and params files are available at http://data.dmlc.ml/models/imagenet/. All models are pre-trained on the ImageNet dataset.
ImageNet is a project which aims to provide image databases for research purposes. It contains more than 14 million images which belong to more than 20,000 classes (or synsets). Most of the pre-trained networks you will ever use will be trained on this dataset.