maxemil

Simply tick the checkbox “Show this to logged in visitors only” This works on a per post / per page basis. Not yet enabeled for custom post types.
You do not need to insert custom functions or shortcodes. You do not need to change theme template files. 
This plugin works by filtering WordPress core functions.
If you “hide” a parent page, the children will be hidden in the menu as well.
Technically the filters are modifying the query or the result of databse calls on standard WP features: wp_get_nav, wp_list_pages, $posts, previous_post, next_post. 
When ticking the checkbox, the plugin saves a key/value pair in the postmeta database table. 
When uninstalling the plugin, the entrys to the postmeta table are deleted.
Suggestions, Questions and feedback are welcome.

Secret Content

Easily mark any post or a page as "for logged in members only", hiding it from public view! (not for custom post types).

bgentry

So, you want to hide one or more of your WordPress posts, so that only people who have the direct link can find them?
Unfortunately, the “Private” option for post visibility makes posts visible only to people who are logged in to the site. But the “Publish” option makes a post visible to the entire world. Brilliant Easy Exclude Posts (BEEP) creates a middle ground by allowing you to make a post hidden.
A hidden post, once published, will be available to anyone with the link, but it will be removed from site feeds.
To make a post hidden, select the check box in the “Exclude this post?” box on the post editor page, and update / publish the post.
There are two settings you can change. Visit your Reading settings page to adjust these settings. First, by default, the post pages will be excluded on every page on your site (except places where the link is hard-coded), but you can adjust the settings to allow hidden posts to display links to other hidden posts in sidebars, such as a recent posts widget. The second option allows you to allow search engines to index hidden pages, whereas by default they are hidden.

Brilliant Easy Exclude Posts

Hide posts from public view so users need the direct link (but not a password) to view the hidden posts. Activate this feature with a simple checkbox  …

Secret Content vs Brilliant Easy Exclude Posts