In development are several extensions of the standard library.
Either as extension or included in the STDLIB with external libraries are:
BINIO.FAS
or stdlib<xx>.arx
ENTMAKE.LSP
INIFILE.LSP
or INIFILE.FAS
REGISTRY.LSP
with stdlib<xx>.arx
or REGISTRY.FAS
(alone)Discussed at various places and provided by others are other modules as well.
The favorites are:
DICT by David Kozina,
COMBINATIONS by Reini Urban, Michael Puckett, David Kozina and Serge Pashkov
GETFILEM and HTREE by Reini Urban
A lot of noise is also about DCL, GEOMETRY, VECTOR, MATRIX and similar functionality. Vector algebra is important and useful. Same with DCL. Matrix algebra is useful but better not to be implemented in AutoLISP because of numeric instabilities. If one doesn't care about that he might implement it by himself or use some contributed functions. I'm still sceptic. Overall, it would be better to provide wrappers to ObjectARX library functions. Geometry is better supported by external ADS libraries which involve stricter licensing issues, but some interesting and advanced geometric libraries are already available.
You might have a look into http://xarch.tu-graz.ac.at/autocad/stdlib/contrib
or http://xarch.tu-graz.ac.at/autocad/adsrx/