MDC Private Message is probably the best plugin to create an easy messaging system in WordPress.

MDC Comment Toolbar enables rich editor toolbar with media uploader in comment field.

MDC YouTube Downloader allows visitors to download YouTube videos directly from your WordPress site.

Allow visitors to choose and preview from available themes from front-end. Different themes for different visitors simultaneously!

A simple plugin to allow authors to upload a banner image. Banner image can be shown anywhere using shortcode or calling a function. Use shortcode [mdc_author_banner_url] or function mdc_author_banner_url(); to return uploaded banner url.

An awesome tool to enable two step verification system on your WordPress site.