Double slit applet:  http://vsg.quasihome.com/interfer.htm

Superposition applet: http://www.colorado.edu/physics/2000/applets/fourier.html

Interference of sound: http://www.falstad.com/interference/

Diffraction - multiple slits: http://www.pas.rochester.edu/~ashiq/java/ex2/

More double slit: http://www.phy.ntnu.edu.tw/java/doubleSlit/doubleSlit.html

Interference from circular waves: http://lectureonline.cl.msu.edu/~mmp/kap13/cd371.htm

and also:  http://www.phy.ntnu.edu.tw/java/waveInterference/waveInterference.html

Superposition of simple wave forms: http://id.mind.net/~zona/mstm/physics/waves/interference/waveInterference1/WaveInterference1.html

Coloured Ripple Tank applet: http://www.falstad.com/ripple/index.html