Datasets for data visualization examples, teaching and research I am searching for existing datasets that we can use to test several datavis techniques we are researching.
I know several resources like those included in R (try plot(Orange) or see here).
But I'd like to take it one step forward:


*

*Which are the best real-world datasets to test a visualization tool?

*Which datasets have you used in academic papers or teaching slides about datavis?

*Which is the best example from the real world to show the advantages of graphing?

 A: I like to use the Anscombe data sets (also available in R) to show the importance of plotting when doing regressions. If you aren't familiar, you get the same regression line and diagnostics from all four data sets, even though the sets themselves all look quite different. You can take the plots below and turn them into residual plots to illustrate problems that you might look for in the residuals after performing a regression.

A: 
which is the best example from the real world to show the advantages
  of graphing?

Any big table.  For examples, google images of "official census table".  You'll see things like the one below.
Also look at Gelman et al. (2002) Let's Practice What We Preach: Turning Tables into Graphs.  American Statistician 56:121-130

A: There are large number of databases available on internet. Depending on the subject, you can get different sources. 
For example, in Human Development subject area you can have data sources at (http://hdrstats.undp.org/):
http://hdrstats.undp.org/en/tables/default.html
For Climate change observation, there is a web with high resolution climate data at (http://www.ipcc-data.org/), for example:
http://www.ipcc-data.org/obs/cru_ts2_1.html
Both examples, contains real data, used in published scientific papers, with large quantity of data. Time related and/or space related data. Visualization possibilities of those data are endless.
A: William S. Cleveland has two books full of great uses of graphics, and the data and code to create the graphs in Visualizing Data is on his website
A: Possibly you already know of these, but here they are anyway:
The UCI Machine Learning Repository has many publicly accessible, real world data sets. 
The US Government makes many of its datasets public at data.gov.
If you want some tricky visualization data, I'd suggest looking at a classification task. Seems to me that the Bag of Words set on the UCI MLR has some nice properties, but I could be mistaken (been a while since I used it).
A: Here are a few.
Sci2 Tool Sample Datasets
http://wiki.cns.iu.edu/display/SCI2TUTORIAL/2.5+Sample+Datasets
Sample datasets that come bundled with Sci2 Tool.
Tableau Sample Data Sets
https://public.tableau.com/s/resources?qt-overview_resources=1#qt-overview_resources
Sample data sets for getting started with Tableau.
Awesome Public Datasets
https://github.com/caesar0301/awesome-public-datasets/blob/master/README.rst
This list of public data sources is collected and tidied from blogs, answers, and user responses. Most of the data sets are free, some are not.
This thread is rather old, hoping this bump will get some new contributions!
A: I just noticed loads of datasets here:
http://www.inside-r.org/howto/finding-data-internet
Don't know if that's any use?
I'm afraid I don't teach visualisation so I can't comment on your specific questions.
A: The datasaurus, available as an R package on CRAN provides a great alternative to Anscombe's data - it is also simulated, but really helped me to teach the importance of exploratory data viz.
It is a set of 12 datasets that all have the same summary statistics despite having radically different distributions - including a fun one that plots as a dinosaur shape. I think it is superior to Anscombe because it highlights that exploratory data viz is not just about identifying how a regression line can be fit, but about exploring the nature of the data.
