FRZYDOOM is a library of ACS scripts for use in custom GZDoom maps in the UDMF format. You can download the compiled library and use it as is, or you download the source code and edit it for your own means. This library is free to download, edit, and share.
This site will list all of the functions available in the library and how you can implement them.
As the library is updated I will attempt to keep this documentation up to date as well. The home page of this documentation will act as a diary for changes to this project as they happen.
To get started using FRZYDOOM, head over here.