Enable oEmbed support for comments. Requires WP 2.9+.
oEmbed is a simple API that allows a website to display embedded content (such as photos or videos) when a user posts a link to that resource. It was designed to avoid copying and pasting HTML from the media you wish to embed.
oEmbed for Comments utilizes WordPress' own oEmbed class, so by default, you can share content from the following sites in your comments:
The plugin allows you to input any URL from one of the listed sites above into a comment.
When the comment is posted and approved, the URL automagically transforms into the embedded content. There is no GUI.
By default, you can only embed content from websites listed on WordPress' internal whitelist. This is to prevent the embedding of malicious content from untrustworthy websites.
To add an oEmbed provider, read the following article for more info: http://codex.wordpress.org/Embeds#Adding_Support_For_An_oEmbed-Enabled_Site
The other option is you can override WordPress' internal whitelist and enable any site that is oEmbeddable by downloading and activating Viper007Bond's Enable oEmbed Discovery plugin. You should only activate the oEmbed Discovery plugin if you trust your user base. You've been warned.