The following URLs are reputed to have source code for NNs. Use at your own
risk. 

 o C/C++
   http://www.generation5.org/xornet.shtml
   http://www.netwood.net/~edwin/Matrix/
   http://www.netwood.net/~edwin/svmt/
   http://www.geocities.com/Athens/Agora/7256/c-plus-p.html
   http://www.cs.cmu.edu/afs/cs.cmu.edu/user/mitchell/ftp/faces.html
   http://www.cog.brown.edu/~rodrigo/neural_nets_library.html
   http://www.agt.net/public/bmarshal/aiparts/aiparts.htm
   http://www.geocities.com/CapeCanaveral/1624/
   http://www.neuroquest.com/ or http://www.grobe.org/LANE
   http://www.neuro-fuzzy.de/
   http://www.cs.cmu.edu/afs/cs/project/ai-repository/ai/areas/neural/systems/cascor/
   http://www.cs.cmu.edu/afs/cs/project/ai-repository/ai/areas/neural/systems/qprop/
   http://www.cs.cmu.edu/afs/cs/project/ai-repository/ai/areas/neural/systems/rcc/
   etc. 

 o Java
   http://www.philbrierley.com/code
   http://rfhs8012.fh-regensburg.de/~saj39122/jfroehl/diplom/e-index.html
   http://neuron.eng.wayne.edu/software.html
   http://www.aist.go.jp/NIBH/~b0616/Lab/Links.html
   http://www.aist.go.jp/NIBH/~b0616/Lab/BSOM1/
   http://www.neuroinformatik.ruhr-uni-bochum.de/ini/PEOPLE/loos
   http://www.neuroinformatik.ruhr-uni-bochum.de/ini/VDM/research/gsn/DemoGNG/GNG.html
   http://www.isbiel.ch/I/Projects/janet/index.html
   http://www.born-again.demon.nl/software.html
   http://www.patol.com/java/NN/index.html
   http://www-isis.ecs.soton.ac.uk/computing/neural/laboratory/laboratory.html
   http://www.neuro-fuzzy.de/
   http://sourceforge.net/projects/joone
   http://joone.sourceforge.net/
   http://openai.sourceforge.net/
   http://www.geocities.com/aydingurel/neural/
   http://www-eco.enst-bretagne.fr/~phan/emergence/complexe/neuron/mlp.html

 o FORTRAN
   http://www.philbrierley.com/code
   http://www.cranfield.ac.uk/public/me/fo941992/mlpcode.htm

 o Pascal
   http://www.ibrtses.com/delphi/neuralnets.html

If you are using a small computer (PC, Mac, etc.) you may want to have a
look at the Central Neural System Electronic Bulletin Board (see question 
"Other sources of information"). There are lots of small simulator packages.
Some of the CNS materials can also be found at 
http://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/areas/neural/cns/0.html