The Regional Players
HomesAway, a Toronto-based company, has an inventory of 65 properties (many of them are not available through other North American agencies) in France, Italy, and Spain. The company's employees regularly visit all of its rentals.
Just France, Custom Italy, and the Italian-focused Homebase Abroad specialize in single destinations, offering farmhouses, luxury villas, and city apartments. All three are renowned for their high standard of service and close relationships with local homeowners.
Abercrombie & Kent's rental agency has a roster of 19 lavish properties in France and Italy with hotel-style service—full staff, daily housekeeping, a groundskeeper—plus access to thousands of other houses around the world.
London-based Carpe Diem Luxury Travel and Five Star Greece are experts in high-end private luxury rentals, most of which aren't found on the open market. The former has properties from Corsica to Austria, the latter on many lesser-known islands.
Also worth a look Fontelunga Villas, LaCure, Rentvillas, Unique Properties & Events, Villas & Apartments Abroad, Villas of Distinction, and Wimco Villas