If you are looking to sell or rent international real estate you may wish to use an international listing service to sell or rent your properties. Most internationally listing sites are ranked on a a mixture of cost versus results versus exposure. To get the best results it is usually when the competition is low due to low numbers of listings on your chosen area, a low listing fee, and a high profile listing service website.
Buyers or renters can look through conventional mls listing options, of which there are many, or use a dedicated international real estate website with a tendency to be towards the international buyer but can also cover the national or regional market place.
With 100,000's of property websites it's hard to know who to count in the top 10 and for which category. Many leading real estate websites are run by newspapers or publishing houses that create large amounts of content quickly and drive top level traffic to their sites using a variety of paid for and organic searches.