Reuven Karasik

This plugin alows you to display a message to all your visitors insode a fancy lightbox. 
features:
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Easy interface and simple settings pages
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All features support any type of client-side code: HTML, Javascript, embed of video or enything you want
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Display your message to all visitors in a pop-up lightbox. Automaticly put there a like button, and you will get lots of likes every week
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Your Lightbox

Display lightbox with your message or like button to all visitors

Sachin Khosla

This plugin will allow you to remove the URL option from the comment box. This prevents automated bots to post spam links on your website. It helps your website retain the SEO Juice. 
This is a lightweight very simple plugin. No Javascript, ajax hack.
<h3>Usage</h3>
Simply activate the plugin and the URL input text box will not appear.

Disable / Hide Comment URL

Disable/Hide Comment URL lets you hide the URL/Website input field from the WordPress inbuilt comments block.

Your Lightbox vs Disable / Hide Comment URL