In no particular order
Dataset sources
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El catastro
Open listings
UCI Machine Learning repository
Wikipedia's List of datasets for machine-learning research
https://en.wikipedia.org/wiki/List_of_datasets_for_machine-learning_research
Zenodo
Url
About
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Open Science
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CERN
Gated listings
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Accessed through BigQuery
Might require payments (pricing)
Datasets
Flickr1024: A Dataset for Stereo Image Super-resolution (on Arxiv)
A collection of small corpuses of interesting data for the creation of bots and similar stuff.
a 6.4 TB dataset of permissively licensed source code in 358 programming languages.
IMDb datasets
Url
Frozen full data from 2017
Search engines
Google Dataset Search
City/Country data portals
Portal de datos de Barcelona
Portal de datos de Madrid
Portal de datos de España
London Datastore
NYC Open Data
NOAA Coastal Science portal