If you have Tableau Public, but don't have any data, things come to a grinding halt! Luckily, this compilation of sites (updated monthly) contain hundreds of different datasets that you can get for free.
If you are searching for a particular type of data, use the filter at the top right hand corner to narrow down your search. Otherwise, just click on a bar to link out to the data itself. The bars themselves represent how easy each sites data is to use in Tableau- higher is better!
You can also download the complete list here.