Long-lived technical material: ObjectARX Tips and Tricks and AutoCAD Exposed. The Outside The Box programming posts live in the archive and are tagged for developers as well.
For developers
ObjectARX, LISP, and AutoCAD internals — notes collected over years of shipping CAD tools.
ObjectARX Tips and Tricks
Environment setup, build hooks, demand-loading, and related ObjectARX notes.
Freebies
Developer-oriented freebies for AutoLISP and ObjectARX.
AutoCAD Exposed
Undocumented commands, system variables, AutoLISP functions, and R14 ObjectARX exports.