ObjectDCL is an open source tool which runs inside AutoCAD (2000+) allowing you to easily create professional-looking dialog boxes for AutoLISP and VLisp routines.
Download from: ObjectDCL

This Czech site (don’t worry, the link is for the English page) contains some useful software, routines, patches and fonts for AutoDesk related products. New uploads have their date highlighted in red so even if you check it once in a couple of months, it’s easy to see what’s changed.
XANADU - CAD files and utilities

This site wants to be “For Everything AutoCAD”. Includes AutoCAD blocks (version 2006) tutorials, LISP routines, shx fonts and hatches. There’s also a forum for discussion and, possibly trouble-shooting. Time will tell if this site strikes a chord with users or if it’ll die away like many others before it.
http://www.cadcorner.ca/

Cadalyst has a gargantuan collection of routines and new ones are added to each month. I suggest that you subscribe to their newsletter or RSS feed. Registration is free.

Leonid Nemirovsky’s “Better than Nothing” routines are considerably better than most. He also has some tips and tutorials that you might find interesting. He has consistently updated them over the years.
http://home.pacifier.com/~nemi/

A set of very useful routines by Asbjørn Værnes includes a block manager, a layer manager and a detailed door insertion tool among others.
« not updated of late »
http://autolisp.3utilities.com/

This is a collection of almost everything for the Cad user - SHX fonts, hatches, LISP routines and Blocks. There’s even an old collection of drivers in case you have some old equipment lying around.
« Not updated since 1999 »
http://www.cadresource.com/library/

You can get a massive 238 free hatch patterns [178 kilobytes]. They have a wonderful collection of routines too.

CadDepot also has a massive collection of blocks to insert into your drawings. Not all of them are freeware though and please look out for blocks that have been created by unlicensed versions of AutoCad. If [...]