KiCad can be considered mature enough to be used for the successful development and maintenance of complex electronic boards. KiCad does not present any board-size limitation, and it can easily handle up to 32 copper layers, up to 14 technical layers and up to 4 auxiliary layers. KiCad can create all the files necessary for building printed boards, Gerber files for photo-plotters, drilling files, component location files and a lot more.
It has functions for schematic capture, PCB layout, a 3D viewer, and also a number of external tools that users have developed. There is a calculator for components, track width, electrical spacing, color codes, and even a 'Perform electrical rules check' feature for analysing circuit faults.
Credit to @mfierst form Pluspora for mentioning this one to me.
See KiCad EDA
A Cross Platform and Open Source Electronics Design Automation Suite