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