One popular (and expensive) system is the Aquamist: http://www.aquamist.co.uk/cp/cp.html

A DIY option is documented here: http://www.dawesdevices.com/water.html

Another DIY page: http://users.frii.com/maphill/wi.html

And one more: http://members.shaw.ca/turbofiero/water_injection.htm

A retail site with some info on the Spearco and Injectronic systems: http://www.h2oinjection.com/

AutoSpeed article about their water spray kit, with suggestion it can be used as an injection system too (and a brake cooler!): http://www.autospeed.com/cms/article.html?&A=0589

http://waterinjection.info/ Page that aims at collecting Information on water and a varietry of injectable Knock reducing combustibles