The Electric VLSI Design System is an open-source Electronic Design Automation (EDA) system that can handle many forms of circuit design, including custom IC layout, Schematic Capture (digital and analog), textual languages such as VHDL/Verilog, and much more.  It has many built-in tools (DRC, ERC, simulators, generators, routers, etc.) and built-in technologies (different CMOS variations, nMos, Bipolar, etc.)  The software is available at www.staticfreesoft.com

The OpenSPARC team has done some initial work with using Electric to generate a floorplan using Electric.

Image of OpenSPARC T1 Chip Level using Electric (Click to Enlarge)

The database file for the complete floorplan is available.