Browser Shots is a free and easy to use tool to quickly get a first insight into the browser compatibility of your Drupal website.
Screenfly is a free and easy to use tool to the the responsiveness of your website on all common devices and screen sizes.
The Drupal CAPTCHA module is a very easy and effective way to avoid spam through contact form, comments and any other submission.
Drupal offers you a very convenient module (Media: TED) to publish videos from ted.com on your website with an Embedded Media Field.
Use the modules "Back to Top" or "Scroll to Top" to allow users an easy way to get back on top of a page (scroll to top button/link)
With the Drupal module Taxonomy Menu you can very easily and quickly create taxonomy menus and automatically keep them up to date.
The Drupal "SEO checklist" module verifies your website for proper search engine optimization and helps you to keep track of what has already been done.
The module Responsive Menus offers you a very easy and quick way to add simple and lightweight responsive menus to your theme.
If you want to use an advanced service to build an active audience commenting your content I recommend you disqus.com
The most common reasons why clean URLs are not working in Drupal and the way to resolve the problems
