Freeware.
http://www.cadencepcb.com/products/downloads/PSpicestudent/default.asp
One of the nicest analog/digital simulator.
http://margo.student.utwente.nl/el/cad-cam/mc5demo.zip
http://margo.student.utwente.nl/el/cad-cam/spice6w3.zip
can be used to model Diodes, Bipolar Transistors, Darlington Transistors, Zeners, Power Transistors, Power Mosfets, JFETs, Small Signal Mosfets, IGBTs and SCRs.
http://www.wssh.net/~wattsup/audio/spicem.zip
Links to free SPICE downloads for a variety of operating systems, including DOS/Win, Windows 95/98/2000/NT/Me, OS/2, Linux, Sun OS, Mac, Amiga, and Atari.
http://www.tu-harburg.de/et1/emv/et1kb/download.html
http://www.ee.washington.edu/circuit_archive/software/spice3f4.tar.gz
a new Windows version of SPICE
http://www.aimspice.com/
32-bit analog, digital, mixed signal circuit design and simulation program
http://www.web-ee.com/Downloads/Simulator/vsd.zip