Some links to data sets

The UCI Machine Learning Repository is one of the oldest and best sources of datasets on the web.

Kaggle is a data science community that hosts machine learning competitions. You can download data from Kaggle by entering a competition. Each competition has its own associated dataset. There are also user-contributed datasets that can be found there.

FiveThirtyEight makes the datasets used in its articles available online on Github.

Amazon Datasets (but you need an Amazon Web Services account, which I think is free.)

Google lists all of the datasets on a page. You will need to sign up for a GCP account, but the first 1TB of queries you make are free.

Data.gov is a relatively new site that is part of a US effort towards open government.

Reddit, a popular community discussion site, has a section devoted to sharing interesting datasets.

Springboard has a list of various data sets, ranging from US Census data to Crime data and the Enron email data.