Machine Learning

    Amazon Web Services Data: http://aws.amazon.com/datasets
    Airlines Data (2009 ASA Challenge): http://stat-computing.org/dataexpo/2009/the-data.html
    AppliedPredictiveModeling (R package): http://bit.ly/16wyvkG
    Australian Weather: http://www.bom.gov.au/climate/dwo/
    Causality Workbench: http://www.causality.inf.ethz.ch/repository.php
    Kaggle competition data: http://www.kaggle.com/
    KDNuggets competition site: http://www.kdnuggets.com/datasets/
    The Koblenz Network Collection: http://konect.uni-koblenz.de/
    Machine Learning Data Set Repository: http://mldata.org/
    Medicare Data File: http://go.cms.gov/19xxPN4
    Microsoft Research: http://research.microsoft.com/apps/dp/dl/downloads.aspx
    Million Song Dataset: http://blog.echonest.com/post/3639160982/million-song-dataset
    More song datasets: http://labrosa.ee.columbia.edu/millionsong/pages/additional-datasets
    MovieLens Data Sets: http://datahub.io/dataset/movielens
    NYC Taxi Data (2010-2013): http://publish.illinois.edu/dbwork/open-data/
    RDataMining.com R and Data Mining ebook data: http://www.rdatamining.com/data
    Social Networking: http://www.cs.cmu.edu/~jelsas/data/ancestry.com/
    UCI Machine Learning Repository: http://archive.ics.uci.edu/ml/
    53.5 billion clicks: http://cnets.indiana.edu/groups/nan/webtraffic/click-dataset

Networks

    Stanford Large Network Dataset Collection: http://snap.stanford.edu/data/

Public Domain Collections

    Data360: http://www.data360.org/index.aspx
    Factual: http://www.factual.com/
    Freebase: http://www.freebase.com/
    Google: http://www.google.com/publicdata/directory
    infochimps: http://www.infochimps.com/
    Quora: http://www.quora.com/Where-can-I-find-large-datasets-open-to-the-public
    RS Collection 100+ : http://rs.io/2014/05/29/list-of-data-sets.html
    Sample R data sets: http://stat.ethz.ch/R-manual/R-patched/library/datasets/html/00Index.html (R!)
    SourceForge Research Data: http://www.nd.edu/~oss/Data/data.html
    StatSci.org: http://www.statsci.org/datasets.html
    UFO Reports: http://www.nuforc.org/webreports.html
    Wikileaks 911 pager intercepts: http://911.wikileaks.org/files/index.html
    The Washington Post List: http://www.washingtonpost.com/wp-srv/metro/data/datapost.html

Science

    Agricultural Experiments: http://www.inside-r.org/packages/cran/agridat/docs/agridat (R!)
    Climate data: http://www.cru.uea.ac.uk/cru/data/temperature/#datter and ftp://ftp.cmdl.noaa.gov/
    Gene Expression Omnibus: http://www.ncbi.nlm.nih.gov/geo/
    Geo Spatial Data: http://geodacenter.asu.edu/datalist/
    Human Microbiome Project: http://www.hmpdacc.org/reference_genomes/reference_genomes.php
    MIT Cancer Genomics Data: http://www.broadinstitute.org/cgi-bin/cancer/datasets.cgi
    NASA: http://nssdc.gsfc.nasa.gov/nssdc/obtaining_data.html
    NIH Microarray data: ftp://ftp.ncbi.nih.gov/pub/geo/DATA/supplementary/series/GSE6532/ (R!)
    Protein structure: http://www.infobiotic.net/PSPbenchmarks/
    Public Gene Data: http://www.pubgene.org/
    Stanford Microarray Data: http://smd.stanford.edu//

Social Sciences

    General Social Survey: http://www3.norc.org/GSS+Website/
    ICPSR: http://www.icpsr.umich.edu/icpsrweb/ICPSR/index.jsp
    Pew Research: http://www.pewinternet.org/datasets/pages/2/
    SNAP: http://snap.stanford.edu/data/index.html
    UCLA Social Sciences Archive: http://dataarchives.ss.ucla.edu/Home.DataPortals.htm
    UPJOHN INST: Search for data at http://www.upjohn.org

Time Series

    Time Series data Library: http://robjhyndman.com/TSDL/

Universities

    Carnegie Mellon University Enron email: http://www.cs.cmu.edu/~enron/
    Keel Repository: http://sci2s.ugr.es/keel/datasets.php
    Ohio State University Financial data: http://fisher.osu.edu/fin/fdf/osudata.htm
    UC Berkeley: http://ucdata.berkeley.edu/
    UCLA: http://wiki.stat.ucla.edu/socr/index.php/SOCR_Data
    UC Riverside Time Series: http://www.cs.ucr.edu/~eamonn/time_series_data/
    University of Toronto: http://www.cs.toronto.edu/~delve/data/datasets.html