Data
While data visualization is fun and creative, you can't expand your abilities as a data designer without new and challenging data. I try to tackle a new data set every week. That being said, data is everywhere and it can be difficult to identify what is usable and what is not, so this is your one stop shop for usable data on the web.
Google Dataset Search
Google finally found a way to solve all of our data search problems by creating a search engine just for datasets. You'll notice that the other web pages included in this list are actually used in the engine, so it's a nice one-stop-shop if you're not sure what you're looking for.
Data is Plural
This is a great google sheet based on the weekly newsletter Data is Plural.
data.world
A community created to share and collaborate on data sets covering any and all topics. You'll need to sign-up to get full access, but it's well worth it.
Kaggle.com
This is a site dedicated to data science in particular, so it has a pretty good running catalog of data sets submitted by people and other entities. I enjoyed the Simpsons and South Park data.
For those that use Tableau, these are Web Data Connectors that have been written by the Tableau Community. Easy access playground data when leveraging Tableau.
Data.gov
In 2009 the US government started opening up their data to the public via a single source website. There is a healthy amount of information on this site ranging from census data to hourly precipitation.
Open Football (Soccer)
A github based repository for data related to "the world's game". Players, teams, standings, wins, losses, etc.
@CoolDatasets
This is a Twitter profile that claims to have the best open source datasets available. The jury is still out for me, but it's an interesting follow.