What are the freeware SPICE simulators available?
ngSpice is available for gEDA. 
gnuCAP is also available for gEDA.
LTSpice is free from Linear Technology.


I thought that one of the other analog chip makers had a spice too but I can't remember
who :(

I have been to a few talks on simulation given by physicists and EEs who have done
chip design. Each of the talks seems to end like this ---


Except for simple circuits you will spend most of your time getting models
    and determining where the models need to be modified for your application.
Unless you are doing work for an IC manufacturer the manufacturer will not
    give you detailed models.
You will not be able to avoid a prototype.
You should only simulate subsections of your design. Simulating the entire
    design is not usually practical.


Also most of the free simulators are not distributed with models. Re-distribution of
the models is usually a copyright violation. LTspice is distributed with models of
the Linear Tech parts. I am not sure the quality of the models. Most manufacturers 
do not want to reveal too many details about their process.