OpenBIM

ARCHICAD
GRAPHISOFT ARCHICAD and COBie 2 (PDF)
IFC Reference Guide for ARCHICAD 18 (PDF)
IFC Reference Guide for ARCHICAD 19 (PDF)
ARCHICAD Connection Add-In for Autodesk Revit

Autodesk
Revit IFC Exporters (Autodesk Exchange Apps)
IFC for Revit (Sourceforge)
ARCHICAD Connection Add-In for Autodesk Revit (Add-In made by GRAPHISOFT, to be installed on Revit user's machine. Creates a tab on the Revit Ribbon under Add-Ins)

buildingSMART
buildingSMART
buildingSMART-tech

COBie
COBie (Overview by Bill East on WBDG website)
GRAPHISOFT ARCHICAD and COBie 2 (PDF)
COBie IFC MVD (Model View Definition)

IFC
IFC (Industry Foundation Classes) (Wikipedia)
IFC2x3 (online Specification - Technical Corrigendum 1)
IFC4 (Release Candidate 3)
IFC4 (Addendum 1)
IFC Reference Guide for ARCHICAD 18 (PDF)
IFC Reference Guide for ARCHICAD 19 (PDF)
What is IFC and what do you need to know about it? (blog.areo.io)

Level of Development
BIMFORUM LOD Introduction
A Level of Development Spec for BIM Processes (article on AECBytes)

NIBS
NIBS (National Institute of BUILDING SCIENCES)

OmniClass
OmniClass

Standards
NBIMS-US V3 (National BIM Standard-United States Version 3, requires registration)
NCS V6 (National CAD Standard-United States Version 6, requires purchase for full documentation)
AEC (UK) CAD & BIM Standards

Viewers - IFC
DDS-CAD Viewer
Solibri Model Viewer
Tekla BIMsight

WBDG (Whole Building Design Guide, a Program of NIBS)