Dogan Ucar

WP Last Modified adds the ‚last_modified_date‘ shortcode to your WordPress installation. This shortcode shows the last timestamp of your post/page. Simply use the „format“ attribute for custom date formats (it uses PHP’s date() function). 
The “description” attribute enables a brief description which changes has been made with the last modification. 
WP Last Modified shows also the actual revision of and the publication date of the post/page. 
You can customize the text which will be shown under each post/page under settings -> WP Last Modified Settings. You can also hide one of the elements if you do not want them.
With version 1.1.0, WP Open Last Modified was renamed to WP Last Modified.
Features:
<ul>
<li>last modification timestamp of a post/page</li>
<li>publication timestamp of a post/page</li>
<li>timestamp format customizable</li>
<li>revision count</li>
<li>custom text with placeholders</li>
<li>last modification description</li>
<li>excluding single posts/pages from displaying the last modification timestamp</li>
</ul>

WP Last Modified

This plugins adds the last modified date, current revision and the publication date of your post/page. Simply use the shortcode [last_modified_date]

shawfactor

A <code>304 Not Modified HTTP Header</code> tells your visitors that nothing has changed since their last visit. This is important as without this header visitors may be downloading your webpages from scratch each time, slowing down their experience and your site.
Generally if you do not pass <code>304 Not Modified HTTP Header</code> through your HTTP header, crawlers will fetch your whole website and then compare the content of your site with the already indexed content to check whether anything new has been updated on your website or not. But this process consumes a lot of your server resource and bandwidth.
You can know more about it <a href="http://www.feedthebot.com/ifmodified.html" rel="nofollow ugc">here</a>.
<h4>Before reading any further</h4>
Before reading any further it is important to understand this plugin uses output buffering, in fact the Etag sent by this plugin is a hash of the sites buffered content. If your site uses server side caching this plugin may break the caching functionality. Therefore I suggest that you please make sure you have studied the FAQ, before installation and activation.

LH Add Headers

Adds the ETag, Last-Modified, and if appropriate 304 headers to HTTP responses generated by WordPress for more efficient caching.

WP Last Modified vs LH Add Headers