Development

ARX LT SDK
An extension to AutoCAD LT to enables the ObjectARX API as with the standard version of AutoCAD.
http://www.cadaware.com/

AutoCAD Developers Group Europe
A non-profit association of software developers who are working on applications around AutoCAD.
http://www.adge.ch/

AutoCAD Exposed
Information about undocumented features of AutoCAD, such as commands, system variables, and functions of the ADS, ARX, and AutoLISP programming interfaces.
http://www.manusoft.com/Resources/AcadExposed/index.stm

ObjectARX & Dummies
Talk about programming using .NET framework and ObjectARX Autodesk API.
http://arxdummies.blogspot.com/

Open Design Alliance
An association of CAD customers and vendors committed to promoting Autodesk's AutoCAD DWG drawing file format as an open, industry-standard format for the exchange of CAD drawings.
http://www.opendesign.com/

Py_acad
A free software module that makes it possible to extend AutoCAD with the Python programming language, in a similar way as with AutoLISP.
http://pyacad.sourceforge.net/

Through the Interface
Site made by Autodesk member that contain information to ObjectARX API development and code samples.
http://through-the-interface.typepad.com/

CADing && Coding
A learning resource for AutoCAD Programming and Customization. AutoCAD.Net programming with C#, VBA Programming, DIESEL Macros and AutoCAD Tips and Tricks. <small>(August 23, 2007)</small>
http://cadingandcoding.blogspot.com