AutoCAD Add-ons - Mechanical
3DPowerTools, Auto-Z, GrafiCalc,  MechSoft, Solid3D

Development Tools
ACIS 3D Geometric Modeler, C4W, Convergence Geometric Modeler, Coventor

Free MCAD Programs
(goes to FreeCAD.com)

Parts and Details
web2CAD, Thomas Register, OhYeahCAD!, 3D ContentCentral, Trace Parts

MCAD Programs
Autodesk Inventor, Autodesk Mechanical Desktop, CATIA, Pro/ENGINEER, Solid Edge, SolidWorks, thinkdesign, Siemens PLM

Sheet Metal Design
I-deas Sheet Metal Design, MechSoft, Pro/ENGINEER-Foundation, Sheet Lightning, Shop Floor Expert 

Specialized MCAD Programs
Cadmatic, FiberSIM, T-FLEX Parametric

Surface Modeling
Alias, Cobalt, Rhino

Tolerance Analysis
DCS, Sigmund, ToleranceCalc

Viewers
3DView, AutoVue, Myriad Viewer, SolidWorks Java Viewer, ST-Viewer

Virtual Prototyping
ADAMS