I have encountered to an interesting and useful website about international property rentals. It seems useful especially for a group vacation. You can rent a very nice place (villa, apartment etc.) for a week or so and stay with your friends : http://www.sunny-rentals.co.uk/
Scroll down on the page to choose the country you'd like to search for a place. I must say prices are very reasonable, especially for my country, Turkey. I'll keep it in my mind to use it for my next vacation.
Importantant warning (Here is my disclaimer): I am not sure whether or not this site is a legitimate business though.. So, be careful if you decide to use their service. I don't have any relationship with them and their business.