ISO 15924 is the International Organization for Standardization's registry of scripts. Each script is identified by a name and four-letter code. The current version of the standard includes 156 scripts.
The International Organization for Standardization (ISO) has appointed the Unicode Consortium as the registration authority for ISO 15924. See Information and documention — Codes for the representation of names of scripts, section A.3.3 for details on the requirements for adding new scripts. Suggestions for additions and corrections can be made from the Unicode's ISO 15924 page.
ScriptSource includes some scripts that are not part of the ISO 15924 standard; these can be identified by the lack of a four-letter code. Many of these scripts are undergoing research and may be added to a future version of the standard.