Cocos2D
Language: Python
Platform(s): Mac OS, Windows, Linux
Around since: March 2008

Cocos2D for iOS
Language: Objective-C
Platform(s): iOS Devices, Mac OS X
Around since: June 2008

ShinyCocos
Language: Ruby
Platform(s): iOS
Around since: April 2009

CocosNet
Language: C#
Platform(s): iOS (MonoTouch)
Goal: to also support other .NET platforms like Windows, Windows Phone and Xbox 360
Around since: October 2009

Cocos2D for Android
Language: Java
Platform(s): Android
Versions: cocos2d-android (code on github), active branch: cocos2d-android-1 (code on github)
Notable: cocos2d-android-1 was branched off of cocos2d-android in October 2010 because some developers were dissatisfied with its slow progress.
Around since: January 2010

Cocos2D for Windows
Language: C++
Platform(s): Windows, Windows Mobile
Notable: this project was apparently short-lived. One big code push, one blog post, no updates since. I believe we can consider this port dead, especially in light of Cocos2D-X.
Around since: April 2010

Cocos2D for Web
Language: Javascript
Platform(s): Web Browsers
Website with online demo: http://cocos2d-javascript.org/
Around since: June 2010

Cocos2D X
Language: C++
Platform(s): iOS, Android, uPhone, Win32, others
Notable: although the project started in July 2010 (from what I can tell) the project was not widely known until November 13th by announcing it on Twitter.
Around since: July 2010