kaggle.com - many great datasets and website data dumps
data.world - Easy simple source. Includes free and premium datasets.
https://www.kaggle.com/datasets - Many datasets about various topics! Wealth of data! Their product looks pretty sweet also, for Python or R programmers.
Check out their 51GB downloadable database of Amazon product reviews.
AWS data exchange - Gotta find a needle in this haystack, but worth a look
https://flowingdata.com - has some interesting datasets.Trends.co