Philip John

Need to make sure that a site is private for all but the administrators? Just activate this plugin.
There are no settings, no configuration – it just forces anyone that isn’t an admin to go to the login page. They won’t see anything of the site at all. Great for hiding sites really quickly.

Simple Site Lockdown

Provides a really simple mechanism for locking down a site so that it's private to all but logged in admin users.

Aleksei Znaev

This is a spam IP blocker. It is free. It marks any new comment as spam automatically when commenter’s IP exists in at least one of ‘.zen.spamhaus.org’ & ‘.bl.spamcop.net’. 
Plugin is based on public DNSBL class. 
<a href="http://www.phpclasses.org/package/6994-PHP-Check-spam-IP-address-in-DNS-black-lists.html" title="DNSBL class on PHPClasses.org" rel="nofollow ugc">Official page of DNSBL class on PHPClasses.org</a>

Spam IP Blocker

Free spam IP blocker according to public DNSBL bases.

Simple Site Lockdown vs Spam IP Blocker