http://vacationrentals.com/
http://cyberrentals.com/EURO/EURO.html
http://www.vrbo.com/vacation-rentals/europe
http://www.realadventures.com/vacati...als-europe.htm
I have some other sites too but they're only in Swedish, so they will not do you much good.
Make sure to look into which country you wanna go to. All European countries are very different and prices vary a lot from country to country.