Salesforce Demo Orgs
This contains a list of signup links for demo orgs depending on which cloud you want to learn.
Most of these can be found via Google, but I guess centralizing them here is easier.
Basic Developer Org
Financial Services Cloud
CloudCraze / B2C Commerce
This one will require you to butter up your AE to get a test license.
Unobtainable outside of SDO for now.