Queensland science centre 'Seeing sound' interactive page allows pupils to play soft, loud, high and low sounds and observe the wave form. Excellent site!
http://www.sciencentre.qld.gov.au/sciencentre/exhibits/permanent/sound/seeing.htm

Software downloads

PC
'Oscilloscope 2.51' freeware (83kb)
http://polly.phys.msu.su/~zeld/oscill.html

'NCH Tone/Waveform Generator'
http://www.world-voices.com/software/nchtone.html

'Mousing' - Tone generator - moving the mouse alters pitch and volume.
http://www.sagebrush.com/mousing.htm


Mac
'WaveWindow 2.7' shareware (153Kb)
http://members.aol.com/raymeow/

'Pitch Fork' tone generator - choose note or frequency.
http://www.soundhack.com/

Dozens of tools for music and sound for Mac users can be found at the CalArts site.
http://music.calarts.edu/~tre/CompMusMac/#gener