Good datasets for intro CS courses? What are engaging sources of data with which students can explore, develop questions, analyze, and draw conclusions in an intro CS course? I'm looking for flat files like XLSX or CSV, or relational databases in an environment that permits selection and export of a flat file. 

Kelly Powers asked (nearly) this question in the CSTA-members listserv in May 2017 and got 14 answers in a couple of days, which I'll consolidate and extend in an answer here.
I'm going to clean this up later, but am posting the verbatim work of the CSTA members so others can chime in with additional bullets. I'm aiming to have one resource per bullet: Resources name, link, annotation, example ideas for projects


How about CSV Data Sets at https://vincentarelbundock.github.io/Rdatasets/datasets.html
Maybe there’s something there which would appeal to MS kids. (Joe Kmoch)
Look at the Mondial database, supposedly based on the CIA's World Fact Book. (https://www.dbis.informatik.uni-goettingen.de/Mondial/) It's in various  formats other than CSV (various relational databases, XML, etc)  but should easily be exported to CSV form ( see http://www.convertcsv.com/sql-to-csv.htm) . It's got dozens of relations, and is sizable enough so that you really do need to query it via a program to answer questions. (Doug H)
There are some links on my BigData webpage ( under #4.) that could be a lot of fun
https://sites.google.com/a/pgcps.org/livecode/big-data/bigdata---projects (Cyril Pruszko PGCPS)
I had fun in the past using First Fleet. See firstfleet.uow.edu.au (Andrew Williams and Simon Carter)
Titanic passenger lists (Simon Carter)
Census at School asks lots of different questions of hundreds of thousands of U.S. students and has data by grade and state. My students have used the data for linear regression and chi-square tests. (Andrea DePaolo)CensusAtSchool data works well with middle schoolers. I had 7th graders measure their height and armspan and add their own data to the CensusAtSchool's set using a googlesheet. (BB) 
In the UK, publicly accessible historic weather data is useful and offers some lovely cross curricular links to science, maths and geography: http://www.metoffice.gov.uk/public/weather/climate-historic/ 
I think something rather more comprehensive in the US is available here: https://www.ncdc.noaa.gov/cdo-web/datasets (Miles Berry)
https://www.data.gov/  (Ty Davis)
Wikipedia has some extractable data https://www.tableau.com/about/blog/2016/2/excavating-data-gems-wikipedia-50966 
(Ken Price in Tasmania)
Tuva labs tuvalabs.com (Jon Mannion) is an entire data exploration environment with curriculum and a browser-based interface that allows students to drag various variables from a data set onto axes, akin to SPSS or Fathom.
http://stats.oecd.org/ has some OECD datasets
(Ken Price in Tasmania)
Google also has a Public Data Directory at https://www.google.com/publicdata/directory and a data exploring tool.
(Ken Price in Tasmania)
What about Bart, A. C., Whitcomb, R., Kafura, D., Shaffer, C. A., &amp; Tilevich, E. (2017, March). Computing with CORGIS: Diverse, Real-world Datasets for Introductory Computing. In Proceedings of the 2017 ACM SIGCSE Technical Symposium on Computer Science Education (pp. 57-62). ACM.
CORGIS Datasets Project
https://think.cs.vt.edu/corgis/  (Francesco Maiorana)


Here are some additional links from my bookmarks


The KDD Cup is an ~annual competition for the public, aimed at grad students and professionals. Past year's competitions are on the website, usually with the data, and middle schoolers could use the data for easier tasks than the KDD Cup challenge posed. 
Kaggle is akin to and overlapping the KDD Cup.
Teenagers like music. The Million Song Data set could be used to compare song length by artist, for example. 
An image makes for a nice large data set and motivates students to understand nested loops, conditionals, the accumulator pattern, and the best-so-far pattern. I have students pick their own image and decide upon their own task for data analysis (e.g., range of brightness or number of pixels that meet a color criterion) and manipulation (e.g. change brightish yellows to magenta.)
CODAP, the Common Online Data Analysis Platform. A project of the Concord Consortium, led by the person who led development of Fathom, a stats software for K-12. 
I wish there was a women's basketball version of this set of links:


NFL plays in CSV: http://archive.advancedfootballanalytics.com/2010/04/play-by-play-data.html
News article about Big Data in sports, reading across the curriculum: https://blogs.wsj.com/cio/2013/02/06/big-data-in-big-sports/
Programmers' discussion thread about handling football data: https://news.ycombinator.com/item?id=5002974

The Social Security Administration offers CSV files of name frequencies by birth gender and year. State-level files are also available by gender and year.
The UC Irvine Center for Machine Learning and Intelligent Systems has the Motherlode Machine learning repository of datasets. You can search by domain (e.g. life science), by the data set size, or by the type of task students would typically do with that data.  
GapMinder has raw data files of World Health Organization data as well as an online tool for exploring the data along with interesting examples. 
Exoplanets databases
The Current Population Survey (CPS) asks 60,000 U.S. households each  month about income and employment. You can download CPS data tables and can create your own table with the CPS table creator
A similar question on Quora
Earthquakes From USGS, https://earthquake.usgs.gov/earthquakes/map/ allows the user to filter earthquakes by location, date, and magnitude and download a CSV. The website has built-in geographic visualization. (Contributed to PLTW CS Principles course by Nathan Nolte)