Rob Scott

Easy to use plugin that adds custom fields to posts to easily add Twitter Card metadata to your posts. This plugin requires Advanced Custom Fields by Elliot Condon.
<h3>Compatibility</h3>
This add-on will work with: 
1. Advanced Custom Fields version 4 and up 
2. WordPress version 3.8 and up

Eazy Twitter Cards For Posts

Easy to use plugin that adds custom fields to posts to add Twitter Card metadata to posts.

Amit Gupta

iG:Twitter Cards enables <a href="https://dev.twitter.com/docs/cards" rel="nofollow ugc">Twitter Cards</a> on your WordPress website to present your website in an extra useful and stylish way when anyone shares your website URLs on Twitter, be it Twitter website or Twitter mobile app.
At present only summary and player cards are supported. Support for other cards will be added in future. You can validate your website’s Twitter Cards using official <a href="https://dev.twitter.com/docs/cards/validation/validator" rel="nofollow ugc">Twitter Cards Validator</a>.
This plugin allows complete customization of Twitter Cards meta tags before output via filters. If you have custom post types and you wish to have Twitter Cards enabled for them too then this plugin allows you to do that using filters to add/remove post types on which Twitter Cards are enabled.
This plugin has been written with WordPress.com VIP compliance in mind, so you can use it as is even if you host your website with WordPress.com VIP.
Requirements: This plugin requires PHP 5.3 or better and is supported on WordPress 3.4 or better. It might work on a lower version of PHP or WordPress but no support would be provided for those platforms.
Pull requests on Github are welcome.
Github: https://github.com/coolamit/ig-twitter-cards
WordPress.org plugin repo: https://plugins.svn.wordpress.org/ig-twitter-cards/
<h3>Plugin Usage</h3>
Using this plugin is fairly easy. Set the options by going to <code>Settings > iG:Twitter Cards</code> in your wp-admin. If you select to customize cards when creating/editing a post or page then a metabox would be available when you create/edit a post or page. There you can tweak the card by selecting card type, adding/changing Twitter username of author, etc.

iG:Twitter Cards

A plugin to enable Twitter Cards for your WordPress website.

Eazy Twitter Cards For Posts vs iG:Twitter Cards