David Satime Wallin

BMRE Domain Monitor is built to keep an eye on the domains you add.
It checks the expiration date and warns you if a domain will expire in 30 days or less (both through email and in the admin interface)
It also has the feature (optional) to suggest domains that are similar to the ones you’ve added and that are still available.
DISCLAIMER: Do not ONLY trust this plugin for your domains. It works well but I can not be held responsible if something should go wrong and your domain does actually expire.
LICENSE:This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
<pre><code>This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. </code></pre>

BMRE Domain Monitor

This plugin is meant to monitor your domainnames and warn you before they expire.

Nerd Warehouse

Allow users to check domain name availability directly from your website. Availability is shown without redirecting users. With the use of widgets you can add the domain search to multiple locations on your website. You can also quickly check domain availability yourself using the admin dashboard widget. This plugin is multisite compatible and free.
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WordPress Domain Search

This plugin allows you to create domain search widgets and check domain availability from the admin dashboard.

BMRE Domain Monitor vs WordPress Domain Search