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.
Dassault Systemes Draftsight: Free (for personal use until 12/31/19) .dwg CAD program with very good functionality, uses most AutoCAD shortcuts and menu commands. DraftSight recently announced that all free versions (2018 or earlier) will cease to run after 12/31/19.
DataCAD forum: Search forum for DataCAD questions & troubleshooting, also includes access to the DBUG archives
Level of Design BIM Forum: This Forum has developed Specifications for BIM Agreements Documents AIA is offering and includes a link to them, free Sample documents and The LOD Specifications they produced.
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