From Ironic Software:
OpenMeta is an OS X Cocoa Objective - C technology that allows any application to read and or write tags, ratings, etc to any file. It works with 10.5 and 10.6
The idea of the project is for commercial, open source and in house programs to be able to use OpenMeta for reading, writing and dealing with user defined meta data. The metadata is stored in extended attributes (xattr). Some of the metadata stored can be stored in such a way so as to trigger the Spotlight indexer to add the data to the Spotlight DB, allowing it to be searchable.