tagging

Dataset of information derived from and related to one million contemporary songs, with more than 50 variables (including information on track metadata, social networks, and more)

Dataset of information about all of the papers presented at the 2015 Neural Information Processing Systems (NIPS) Conference

API for Flickr image service, allowing access to a wider range of Flickr data, including users, groups, photo comments, photo geolocation, and more. (See also the 100 Million Flickr Images project.)

Database of images tagged with a hierarchical labeling structure, including information on image characteristics

Various Twitter dataset collected for academic studies (largely focusing on news)

Dataset of timestamped tweets and corresponding demographic information about authors (i.e., gender and location)

List of lexicons for word-emotion, word-sentiment, and word-color associations derived from a variety of sources (including Amazon, Yelp, Amazon Mechanical Turk, and Twitter)

Crowdsourced dataset of associations between words and emotions and valence (in English and other languages), with some visualization tools

List of music-related databases

Database of Creative Commons licensed music reviews; linked to MusicBrainz