10 Downing Street,
or use the contact page Email the Prime Minister’s Office.
To let Turkey know how you feel about their collaboration with the flotilla, the PM's office can be reached here firstname.lastname@example.org.
To let France feel a little heat, the president himself can be reached at http://www.elysee.fr/ecrire/
Of course, there's no one quite like the United Nations for Israel-hating. Let them know they can't get away with it anymore by writing to http://www.un.org/en/contactus/
The president of the United States can be reached through this page http://www.whitehouse.gov/contact, while American citizens can contact their representatives easily through this page http://www.house.gov/, while senators can be located here http://www.senate.gov/.
The president of the European Union, which has already weighed in on the side of the flotillists, long before all the facts were in, can be contacted through its president email@example.com or its ombudsman http://www.ombudsman.europa.eu/shortcuts/contacts.faces. EU citizens can find addresses for their MEPs through the site as well.
To criticize media coverage of the incident, there is always the venerable NY Times letters page firstname.lastname@example.org. Some of the nastiest anti-Israel coverage has come from The Times and The Guardian, two of Britain's largest newspapers. Give them a piece of your mind to the Times at email@example.com and the Guardian at firstname.lastname@example.org. Nastier still is the Guardian's sister publication, The Observer email@example.com. By far the vilest American reporting on the incident has been at Salon.com. Let them know how you feel about this here http://www.salon.com/about/letters/.
Again, be polite, be strident, be strong. And thank you.