AutoCAD is the most used computer program for people who need to create 2D (and 3D drawings), especially in industries such as architecture and engineering. Being that it is the industry standard, it is also quite expensive…
BRL_CAD is a powerful cross-platform (Windows, Mac, Linux) open source solid modeling system that includes interactive geometry editing, high-performance ray-tracing for rendering and geometric analysis, image and signal-processing tools, a system performance analysis benchmark suite, libraries for robust geometric representation, with more than 20 years of active development.
LibreCAD is a free Open Source CAD application for Windows, Apple and Linux. Support and documentation is free from our large, dedicated community of users, contributors and developers. You, too, can also get involved!
FreeCAD is a parametric 3D modeler. Parametric modeling allows you to easily modify your design by going back into your model history and changing its parameters. FreeCAD is open source (LGPL license) and completely modular, allowing for very advanced extension and customization.
FreeCAD is multiplatfom, and reads and writes many open file formats such as STEP, IGES, STL and others.
gCAD3D. ok, it's in German, but don't freak out! This application includes integrated 3D – OpenGL display, plus all the “basics” of a computer design program. Available for Windows and Linux.
QCad. Cross platform 2D. Probably the simpler one to use.