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

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.

.htm on PAGES vs Custom Page Extensions