Simply put, redtamarin is based on the Tamarin project (eg. the Flash Player Virtual Machine) and allow to run AS3 code on the command-line.
But because Tamarin only support the AS3 builtins (Object, Array, etc.), redtamarin add numerous native functionalities, so you can test if your program run under Windows or Linux, or read/write files, or send data with sockets, etc.
Put another way, if Adobe AIR allow you to build desktop executable with a GUI, redtamarin allow you to build desktop executable with a CLI.
To support the use of the AS3 language for cross-platform command-line executable,
as single exe either for the desktop or the server, as scripts for automation,
as tools for the Flash Platform community.
This project is active even if the sources have not been updated recently.
One of the first questions people ask when discovering redtamarin are
Because of that we decided to stop implementing new native functionalities for a while and focus instead on "building things with it", and here what we got so far:
Official Website
http://redtamarin.com
RedTamarin Google+ Community
https://plus.google.com/communities/111596731002136630496
File a Bug Report or a Feature Request
https://github.com/Corsaair/redtamarin/issues
Our original repository was hosted on Google Code
http://code.google.com/p/redtamarin/
Now we host the sources on our own server and mirror on Github.
Not that we don't trust Github but we have been burned too many times by open source repositories closing down on us