I use the free Protel Autotrax. This has been public domain for three orfour years. It is much better than the older Public Domain Protel Easytrax in almost every way. Easytrax has been public domain for at least 10 years. http://www.protel.com/downloads/files/atx161nd.exe http://www.protel.com/downloads/files/easytrax.zip
Autotrax has no limitations, reads net lists, has an auto router, and does drc checks.
There is a GNU linux layout package http://bach.ece.jhu.edu/~haceaton/pcb/
which a friend of mine supports.
Both of these programs generate gerber files. Autotrax / easytrax do not generate 274x embedded aperture gerber files which many board houses require, so I wrote an application to merger the aperture files and 274d gerber files into 274x gerber files.