LTspice is available free from http://www.linear.com/
This simulator is superb, and suitable for all designers, from beginners to 'advanced'.. It is well suited to simulation of Theremin circuits.
The best tutorial I have found to get you started can be downloaded at
This zip file contains a PowerPoint presentation SWCAD.PPT and running this teaches you all the basics you need start using LTspice.
The Full reference manual is here: http://ltspice.linear.com/software/scad3.pdf
A few good guides:
http://web.cecs.pdx.edu/~ssuzuki/engr_notes/ltspice/LTSpice_help.pdf (A good tutorial, particularly on setting up simulation types and creating subcircuits.)
http://pages.suddenlink.net/wa5bdu/ltguide.pdf (useful tutorial - data on transformers..)
http://home.mindspring.com/~eeweb/ltspicesubckts.pdf (more advanced sub-circuits)
