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14K REST APIs for all sorts of stuff: http://www.programmableweb.com/
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Twitter, Facebook, etc, see Mining-the-Social-Web-2nd-Edition
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Weather: http://openweathermap.org/api
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https://www.quora.com/Where-can-I-find-large-datasets-open-to-the-public
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http://readwrite.com/2008/04/09/where_to_find_open_data_on_the
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http://blog.bigml.com/2013/02/28/data-data-data-thousands-of-public-data-sources/
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http://www.secureworks.com/resources/blog/5-valuable-contextual-data-sources-for-small-businesses/
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Honeypot: http://security.stackexchange.com/questions/10168/setting-up-a-honeypot
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Government: http://www.data.gov/
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Finance: http://quant.stackexchange.com/questions/141/what-data-sources-are-available-online
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Data mining-related: http://www.kdnuggets.com/datasets/index.html
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Health care: http://www.ahrq.gov/research/data/dataresources/index.html
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Arts: http://arts.gov/grants-organizations/research-art-works/publicly-available-data-sources