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Software

Circuit Simulation

LTSpice/SwitcherCAD, Great, free circuit simulation tool - http://www.linear.com/designtools/software/
MacSpice - Spice on the Mac http://www.macspice.com/

Eagle - http://cadsoft.de/
- Very good cross platform support. Restricted use evaluation version is free.
geda - http://www.gpleda.org/
- EMSL is a big advocate of this tool.
gerbv - http://gerbv.sourceforge.net/
- Part of the geda suite, incredibly useful for checking your gerbers before sending them out!
Kicad - http://www.lis.inpg.fr/realise_au_lis/kicad/
- Support for Windows and Linux. Free and open source. Includes 3D rendering support via Wings3D(GNU).

"Open Symbol Project" with Mark Whitis - “The Open Symbol Project aims to create an open collection of schematic symbols and PCB footprints released under a license that does not interfere with use in either commercial or free projects.”
PCB Matrix has two tools that are useful for creating land patterns for SMT components (Windows only)
- LP Viewer, LP Calculator: http://www.pcbmatrix.com/Downloads/LPSoftware.asb
Open Circuits: PCB footprints lists a bunch of PCB footprint libraries you can download.