An entry level description of various datasets and their accessibility.
Datasets housed within sklearn - http://scikit-learn.org/stable/datasets/index.html
(examples - mnist number images, boston housing prices, iris types, Titantic survivors)
Kaggle - https://www.kaggle.com/datasets
(examples - credit card fraud, world happiness index, horse colic)
UCI Datasets - https://archive.ics.uci.edu/ml/
(examples - wine quality, forest fires)
montreal - http://donnees.ville.montreal.qc.ca/
nyc - https://opendata.cityofnewyork.us/
usa open data - https://www.data.gov/
health - https://www.healthdata.gov/
education - https://www.data.gov/education/
sustainability - https://cs.stanford.edu/~ermon/cs325/
mscoco - http://mscoco.org/
(300,000+ images, image recognition, human-keypoints, segmentation, and captioning dataset)
CIFAR10/CIFAR100 - https://www.cs.toronto.edu/~kriz/cifar.html
(Large dataset of 32x32 color labeled images with 10/100 labeled classes)
IMAGENET - http://www.image-net.org/
(Image database organized according to the WordNet hierarchy)
Described by objective - https://deeplearning4j.org/opendata
List of potential projects and datasets in computational sustainability : http://cs.stanford.edu/~ermon/cs325/