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.