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.

Discount Web Designer

Appends .htm to the URL of PAGES when using pretty permalinks
Check out more information here http://www.discountwebdesigner.com/wordpress-plugins/htm-on-pages.htm

.htm on PAGES

Appends .htm to the URL of PAGES when using permalinks

Custom Page Extensions vs .htm on PAGES