@Anywhere supports auto-linkification of usernames, hovercards (similar to ones we see on Twitter.com), follow buttons, tweet boxes and user login/signup options (a.k.a. Twitter Connect). All of these tools are meant to keep visitors Twitter-happy while on third party sites.
Ultimately Twitter hopes that publishers will find the integration options simple and choose to use the tools to keep site visitors more engaged. Of course they also hope to encourage more Twitter activity and get users to share more content back with Twitter. In laymen terms, @anywhere is simply Twitter's answer to Facebook Connect.
Since the initial announcement, Twitter has been working with a smattering of companies pre-public launch, including Mashable, The Huffington Post and Citysearch. They hope these early implementations will help drive home the utility of Twitter integration. Online publishers can pick up the developer tools at Twitter's new developer site starting today.