I'am looking for online flight booking engines in United States and
United Kingdom? please provide me not only most popular, i need all
that you can find

Hi! Thanks for the question.

The following are online flight booking engines I found for the UK and
the US.

UK

Cheapflights
http://www.cheapflights.com/

Cheep Flights
http://www.cheepflights.com/

Expedia UK
http://www.expedia.co.uk/Default.asp?Redir=1&FromPage=http://www.expedia.co.uk/Default.asp?Redir=1&FromPage=

Travelocity UK
http://www.travelocity.co.uk/?HPTRACK=ukhttp://www.travelocity.co.uk/?HPTRACK=uk

Ebookers.com
http://www.ebookers.com/about_us/contact_us_comp.html 

Bookmea Cheap Flights
http://www.bookmeacheapflight.com/

Airfares Uk
http://www.airfares.co.uk/

Travel Select
http://www.travelselect.com/cgi-bin/starter.cgi

Farebase
http://www.farebase.net/

Last Minute
http://www.lastminute.com/lmn/pso/catalog/Category.jhtml?CATID=3

Major Travel
http://www.majortravel.co.uk/cgi-bin/farechk.cgi

Skydeals
http://www.skydeals.co.uk/servlet/flightshop/startpage

TISS UK
http://apps.flights.com/flights/CSFlightInfo.jsp?tab=UK 

Travel Store
http://www.travelstore.com/

Go-Hop
http://www.gohop.com/

Just The Ticket
http://www.justtheticket.co.uk/

Marbus
http://www.marbus.co.uk/Travel_Booking/travel_booking.html 

Blue Dog Travel
http://www.bluedogtravel.co.uk/

Travel Tomorrow
http://www.traveltomorrow.co.uk/

Airline Network
http://www.airline-network.co.uk/Homepage/Homepage.asp

Bargain Holidays
http://www.bargainholidays.com/home/home.asp

Cheap Holidays 123
http://www.cheap-holidays-123.co.uk/

Cheap Holidays Now
http://www.cheap-holidays-now.co.uk/

Cheaper Holidays
http://www.cheaperholidays.com/

Flight24
http://www.flight24.co.uk/

Flightline International
http://www.flightlineflights.com/

Global Flightline
http://www.globalflightline.com/

Holiday Travel Direct
http://www.holidaytraveldirect.co.uk/

Kwikfly UK
http://www.kwikfly.com/startgbr.asp

My Travel
http://www.uk.mytravel.com/cgi-bin/mytravel/Homepages/pages/home.jsp

Opodo
http://www.opodo.co.uk/otpbvpl/Homepage/Page/Homepage.jsp?Locale=uk

Teleticket
http://www.teleticket-flights.co.uk/

This is Travel
http://www.thisistravel.co.uk/travel/home/index.html?in_page_id=1

Thomas Cook
http://ww1.thomascook.co.uk/bold/homepage.htm?CC=1

The Travel Lounge
http://www.thetravellounge.com/

Unijet
http://www.unijet.com/

Villaclick
http://www.villaclick.com/

Virgin Holidays
http://www.virginholidays.co.uk/

A2Btravel
http://www.a2btravel.com/home/home.asp 

Air 2000
http://www.air2000.com/

Aer Lingus
http://www.aerlingus.com/cgi-bin/obel01im1/index.jsp 

---------
US

Expedia
http://www.expedia.com/Default.asp

Travelocity
http://www.travelocity.com/

Uniglobe
http://www.uniglobe.com/

Travelnow
http://www.travelnow.com/airlines/search.jsp?cid=496

Hotwire
http://g2.www.hotwire.com/flights.jsp?siteID=A3pLdyET4mI-eSZQ8R9uJPcQh5y4dFq6Ew

Priceline
http://www.priceline.com/travel/airlines/lang/en-us/itinerary.asp?refid=LSA3pLdyET4mI-ViXkhhF1I5e8d2rZ7BmXUA

Flights.com
http://apps.flights.com/flights/mainPage.jsp?tab=DC 

OneTravel
http://www.onetravel.com/index.cfm?DK=1000004012&SessionID=2DDA2EB1-126D-11D7-A84B009027BBDC6E&Token=NULL&Lang=English&Style=2&Modules=Y|Y|Y|Y|Y|Y|Y|Y|Y|Y|Y|Y|Y&

Orbitz
http://www.orbitz.com/App/Landing?placementID=1&pageID=4&siteID=JebwPXM%2FPvA-s9uB0Jyc%2AHiJRcZaiDWs%2FA

SkyTours
https://www.dotres.com/meridia?posid=8YD9&page=mainMenuMessage_reservation&action=requestAir

e-Travel
http://www.e-travel.com/

SideStep
http://www.sidestep.com/?SbAutoOpen=1

Economy Travel
http://www.economytravel.com/?AID=13957&PID=1128854

Cheap Airlines
http://www.cheapairlines.com/?AID=8204826&PID=1128854

4Airlines
http://4airlines.com/affiliates.html?AID=1522768&PID=1128854

11th Hour Vacations
http://www.11thhourvacations.com/index_home.php

Discount Flights
http://www.etntelephone.com/discountflights/

HotFares
http://www.etn.nl/hotfares.htm

Lowest Fare
http://www.lowestfare.com/

TISS USA
http://apps.flights.com/flights/CSFlightInfo.jsp?tab=US 

Worldwide Air
http://www.worldwideair.com/

Trip.com
http://www.trip.com/trs/trip/home/index_01.xsl

Travel Zoo
http://www.travelzoo.com/

Cheap Tickets
http://www.cheaptickets.com/trs/cheaptickets/home/index_01.xsl

Cheap-Airfare.com
http://www.cheap-airfare.com/

Flyaow
http://www.flyaow.com/

Fly Cheap
http://www.flycheap.com/flightsearch.asp?type=RT&outocity=&outdcity=&outyear=&outmonth=&outday=&outtime=&inocity=&indcity=&inyear=&inmonth=&inday=&intime=&triptype=&codeflag=&afflnum=8

Frugalflyer
http://www.frugalflyer.com/

Fly 4 Less
http://www.fly4less.com/

Lowest Fare
http://www.lowestfare.com/

The Travelsite
http://www.thetravelsite.com/

Travel Hub
http://www.travelhub.com/

Travelers Net
http://www.travelersnet.com/

Travel Worm
http://www.travelworm.com/home.asp?ccr=yes&

1-800-FlyEurope
http://www.1800flyeurope.com/

Kwikfly US
http://www.kwikfly.com/startusa.asp

LanCHile USA
http://www.lanchile.com/english/us/

Wander Planet
http://www.wanderplanet.com/

Lets Go Vacation
http://www.letsgovacation.com/en_bookairfare.html 

James Travel Points
http://travelpoints.com/services/

The Storemaker
http://www.thestoremaker.com/travel.html

Airplan
http://www.airplan.com/

Search terms used: 
Online flight booking engines

I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.

Thanks for visiting us. 

Regards, 
Easterangel-ga
Google Answers Researcher

From personal experience, I know also of: 

  www.traveljungle.com  Can be used for US and within Europe.
    US website:        http://www.traveljungle.us/
    German website:    http://www.traveljungle.de/  (in German)

  http://travelbuzz.net/index.htm  For domestic US and international flights.


Of course, all the US and UK airlines also have flight booking engines.