Universal login throughout all Stack Exchange sites
4980
One login for all the SE sites
58686
Logging out of Stack Overflow or any Stack Exchange site?
110540
Add the ability to cite questions and answers on all Stack Exchange sites
80844
Mapping of multiple server logins to one database user
17481
How to log out of a Stack Exchange site?
81286
Allow "Save And Copy Profile To All Stack Exchange Accounts" to exclude selected sites
17374
Better HTTPS support for Stack Exchange sites
80189
How do I create a multisite with  shared users and nodes?
67607
How to redirect all requests to wp-login.php?
739
Are we creating too many different Stack Exchange sites?
95040
How to use different domains for multilingual sites and keep users logged in?
19435
Can I use the Stack Exchange concept in my own sites?
151770
How to add a Login / Logout link in Main navigation menu?
91308
Allow Documentation in other Stack Exchange communities
34512
How do I hide my accounts on a specific Stack Exchange site?
54930
Where is the login with Stack Exchange option gone?
25280
Two factor authentication for Stack Exchange