Konrád Koller

Do you like to write long articles with many headers? Then this is the perfect plugin for you. This plugin will add a link next to your headers so your user can share your posts pointing to specific parts. Plus a smooth scroll script is also included which will make the overall experience even better.
The package contains a predefined style for Git-like link display but it is hardly recommend to change the style to fit your needs (and your design).
The plugin uses internal cache which may interfere with other cache plugins, if you experience anything strange feel free to contact the plugin author.

ONet Header Linkifier

For advanced users! Github-like header parser for your posts, pages and custom contents. Also lets you fetch TOC for any content.

andreu

This plugin automatically adds HTML anchor tags to plain text links and email addresses embedded in the content of posts and pages (WordPress already does this by default for comments).
In the plugin configuration page, you can choose to open the links in a new window or in the current window.

AutoLink

This plugin automatically adds HTML anchor tags to plain text links and email addresses embedded in the content of posts and pages.

ONet Header Linkifier vs AutoLink