See http://stackauth.com for more details:

This is a domain for centralized services across the entire Stack Exchange network.

In addition to site enumeration for the StackExchange API, StackAuth also serves as the storage domain for our global authentication scheme.

Due to the unreliability of third-party cookies in modern browsers, our scheme uses HTML 5's localStorage to store login credentials under the StackAuth domain. Browsers supported by global authentication: IE8+, Chrome 4+, Safari 4+, FireFox 3.6+, Opera 10.61+.