I've used FreePCB for a number of projects and I think it's very good, plus there's an active user forum. There's also an autorouter that's accessible via the Internet.
FreePCB takes input in the form of a Pads-PCB netlist, though you can also add parts and traces manually.
A lot of software for electronics can be found here: http://www.educypedia.be/electronics/easoftsim.htm
Both demo's of commercial as freeware are available.
More software subjects are also available on these pages: