GrandSlambert

Allows you to add a custom extension to your page URLs. This only affects pages created under the Pages menu and not your posts, category or tag URLs.
Features include:
<ul>
<li>Select which extension you wish to use on all pages.</li>
<li>Adds an item to the Pages Menu to set the global extension you wish to use.</li>
</ul>

Custom Page Extensions

Allows you to add a custom extension to your page URLs. This only affects pages created under the Pages menu and not your posts, category or tag URLs.

debasispattnayak

Appends .htm to the URL of PAGES when using pretty permalinks and swaps dashes with underscores
Check out more information here http://devcreations.com/wordpress-plugins/htm-n-underscores-on-pages.html
DISCLAIMER: This plugin has been heavily tested, however it does require above average technical knowledge. Please use it at your own risk and do not forget to back up your files and databases beforehand with other backup system. Please do not attempt to use the plugin if you’re new to WordPress or have a limited technical background. Please seek out professional help if your in question of anything.

.htm and underscores on PAGES

Appends .htm to the URL of PAGES when using permalinks and swaps dashes with underscores

Custom Page Extensions vs .htm and underscores on PAGES