LibreCAD (originally QCad then CADuntu) is an open source 2D CAD application for Windows, Apple and Linux. It is based on Qt, which is a leading cross-platform application and UI development framework.
LibreCAD can read DWG files (and others) as of the latest Nightly build. It writes DXF files, but can also export SVG, JPG, PNG, PDF and other files. It has layers, blocks, splines, polylines, ellipse tools, advanced tangent line & circle tools, transformation tools, an advanced snapping system and more. Because LibreCAD focuses on 2D geometry it is fast and lightweight, with an installer of less than 30mb.