Easy Mentions allows the users to link to other users' profiles in posts (by using @username) and tags (by using #tag), basically like Twitter.
Just make a new post, write the content with a @username (can be any username) in the text. When you submit the post, the plugin will automatically link the usernames (which exist) to their profile. You can also link the tags in the same way by using #tag. Note that the plugin doesn't link the usernames or tags with spaces.
You can also disable linking of users or tags by going to the settings page.
The plugin can also add a Reply link below each post, which when clicked, adds the reply text (configurable) in the post textbox. This can be enabled via the settings page, but before that please see #1 question in the FAQ.
