Salesforce's site is a great place to learn about all things Salesforce. We are fans of help.salesforce.com too. It is great place to locate documentation.
Trailhead is an admins go to for self pace online learning.
Salesforce Admin is a great resource to connect and learn from other Salesforce Admins. We are big fans of the Salesforce Admin podcast.
ShellBlack is another consulting firm and they have some great Whiteboards to help Admins learn about Salesforce features.
Security Let's Not Fight It is a blog we find helpful for people trying to better understand data sharing!
What should you do when an admin leaves? Check out this "Poof Your Admin Has Disappeared Now What" Blog
How do I get certified? Check out this page!
Need some video tutorials? Check out Let's Play Salesforce on YouTube!
Not a member of the Salesforce Trailblazer community? You should join the fun here!
Did you know there's a community dedicated to developers? Check it out here!