Mdkart

This plugin for WordPress automatically add the rel=”lightbox[ID-OF-THE-POST]” to images linked in a post 
ID-OF-THE-POST is unique per post so all images per post are grouped in one lightbox set. 
Works also with the gallery Fonction of WP 2.7 and in comments
Doesn’t add the the files required for Lightbox (it only add the rel lightbox tag)! You have to insert yourself in your theme the required files or use a plugin like Lightbox 2 WordPress Plugin or Lightbox 2.3 plugin for WordPress
<h3>Changelog / Todo</h3>
0.4.1 
* Optimizing the code 
0.4 
* Now Compatible with the Gallery Fonction of WP 2.7 
* Adds also rel tag in the comments 
0.3 
* Correction of a bug that added rel tag on links done on img tags. 
0.2 
* Correction of a small bug described by Kristof Mattei. Thanks to him for the help!
Thanks to all who sent bug reports and ideas for improvements. 
Please send me a mail if I forgot you to mention here.

Add Lightbox

This plugin for Wordpress automatically add the rel="lightbox[ID-OF-THE-POST]" to images linked in a post

jschuller

Easily embed video within your posts. Adds a widget to the posting screen that enables you to post videos by simply providing the URL to the video hosted on common video sharing websites.
<h3>Usage</h3>
<ol>
<li>Within the WordPress Administration Control Panel, expand the Posts tab and then select Add New.</li>
<li>Create a title for your post and add a description within the post field.</li>
<li>Paste the URL to the video you wish to embed within the Video URL field.</li>
<li>Enter the Width and Height parameters for your video within the fields provided (optional).</li>
<li>Click Update Post to save your options.</li>
<li>Once the page reloads, a full preview of your embedded video should be displayed.</li>
</ol>

Simple Video Embedder

Adds a widget to the posting screen that makes posting videos a cinch.

Add Lightbox vs Simple Video Embedder