This is a list of some cool WordPress plugins for December 2011 :-
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
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 plugin will help you to manage, track and control file downloads from your site.
Posts 2 Posts allows you to create many-to-many connections between posts of all kinds.
wpStoreCart is a free, open source, and full featured e-commerce platform.
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 makes it easy to manage custom columns for your posts, pages, custom post types and user screens.