CAD Depot.com: "the ultimate CAD shareware directory" - hard to argue with their tag line.
Calculators On-Line: Jim Martindale's incredible list of 16,740 downloadable specialty calculator programs, tools and sites. Also links to his encyclopedic Reference page.
Cheap Tricks: Extensive DataCAD resources including the best newsletter on DataCAD and many other Digital programs and concepts.
DataCAD forum: Search forum for DataCAD questions & troubleshooting, also includes access to the DBUG archives
DBUG Archives: Search the Archives of the DataCAD Boston User Group forum, probably the best CADD FAQ ever. If you have a question or problem with any version of DataCAD or its macros, you will likely find the solution discussed in detail here. This forum is monitored by the developers of the software, who sometimes contribute.
Free Brava .dwg viewer: The Informative Graphics Corp. that makes the very useful Brava line of Tiff, PDF, and other format viewers has come out recently with a FREE DWG viewer.
Revit City: Independent site for Revit information. Forum, Revit Component downloads (shareware) and other helpful resources.
SketchUp web site: The easiest and least expensive way to get into 3D modeling. Now owned by Google, there are free and professional versions