S Hetjens

This plug-in restricts access to blog and feed. All anonymous visitors will be forwarded to the login page of WordPress 
before accessing the blog content. If access is restricted, rss and atom feeds will be disabled, too.
The feed can be activated as a private feed for each user. Every user will have a unique feed url to access it. That 
url is based on the username and an md5 hash of the hashed password stored in database. This plug-in will modify all 
by wordpress inserted feed urls to the user specific ones. But be carefull. If you content is confidential you should 
not activate the feed. There is no way to check which services are reading (and maybe publishing) the feed’s content.

Hetjens Registered Only

This plug-in restricts the access to blog and feed. Visitors need to login before accessing the blog. It offers a private feed for every user.

curlybracket

This WordPress plugin restricts non-authenticated users to a number of pages to view. You can define how many pages an unauthenticated user may view. If the limit is reached, the user is redirected to the WordPress login page (default) or a page of your choice.
This plugin uses cookies, but not only. People can not just empty their cookies and start watching your page again. In this plugin i use a technique in which I combine a users’ IP address with the user agent of this particular user, and create a more or less unique hash out of these two informations. This hash is stored as a file in the wp-content/uploads directory.
A site administrator can choose to reinitialize all sessions and empty the hash files.
Furthermore, robots might be blocked. However, important bots like GoogleBot, BingBot and YahooBot are excluded from the restriction.
This technique might be problematic for users using Tor / TorBrowserBundle as lots of people share a same originating IP address and also the same user agent (check here https://panopticlick.eff.org/). I will certainly test this aspect a little better in a later version of this plugin.

Pageview content restriction

Restricts access to your Wordpress site after a certain number of pageviews.

Hetjens Registered Only vs Pageview content restriction