I'm looking for features in websites that I may not have seen before that are unique, and are excellent features that not only improve the user-experience, but help the surfers come back for more.
- StackOverflow's status update banner (seriously now this is awesome.)
- RSS feeds (when they were new)
- Hotlinks to social networks (facebook, twitter, etc..)
- Auto-posting to social networks (popup and auto-post link/content)
- Mobile phone integration
- OpenID login passthrough
- AJAX push/pull technology for content updates
- tinyUrl and others that concatenate URLS shorter.
I can think of many more, but if there is something you've seen that is unique to a site or isn't very well known, I'd love to know.