This is a list of some cool WordPress plugins for December 2011 :-
Social
Social allows you to broadcast posts to Twitter and/or Facebook, pull in items from each as comments, and allow commenters to use their Twitter/Facebook identities
Scripts n Styles
Scripts n Styles allows Admin users the ability to add custom CSS (at the bottom of the head tag) and JavaScript (at the bottom of the body tag) directly into individual Post, Pages or any other registered custom post types.
Fast Secure Contact Form
This plugin lets your visitors send you a quick e-mail message and blocks all common spammer tactics. Additionally, the plugin has a multi-form feature, optional extra fields, and an option to redirect visitors to any URL after the message is sent.
Download Manager
Download Manager plugin will help you to manage, track and control file downloads from your site.
Posts 2 Posts
Posts 2 Posts allows you to create many-to-many connections between posts of all kinds.
wpStoreCart
wpStoreCart is a free, open source, and full featured e-commerce platform.
Email Address Encoder
Email Address Encoder is a lightweight plugin to protect email addresses from email-harvesting robots by encoding them into decimal and hexadecimal entities.
Codepress Admin Columns
Codepress Admin Columns makes it easy to manage custom columns for your posts, pages, custom post types and user screens.