If you want a completely silent keyboard you would have to go for one of the touch models, such as
http://www.rsi-shop.co.uk/products/moskey-touch-keyboard-and-mouse-usb.asp
Incidentally as standard this does emit a 'click' sound when keys are touched, but you can turn the sound off. With sound off a light will flash instead.
There is a user manual for this keyboard at
http://www.volamp.com/Moskey/MoskeyManual329.pdf
There is more information from the manufacturer here
http://www.volamp.com/Moskey/Datashet.htm