Seasonz Travel, best known for its collection of luxe New Zealand retreats (Huka Lodge, Kauri Cliffs), also rents private lodges in the country (as well as islands, such as Great Mercury).
Ampersand Travel, a London-based agency, has a villa collection in Sri Lanka that includes lakeside bungalows, Balinese-style beachfront houses, and restored 18th-century merchant homes, all of which are fully staffed.
Unique Properties & Events doesn't advertise and doesn't list any details of its 120 properties in 35 countries (including Brazil, Morocco, and Kenya). Instead, the company prepares presentations, based on the specifications of each client.
New York's Villas & Apartments Abroad appeals to demanding high-end travelers. The company places clients in houses throughout Australia, Thailand, South Africa, the Seychelles, and Europe.
Sanctuare, synonymous with over-the-top private retreats, has a portfolio that includes Richard Branson's Necker Island and Kasbah Tamadot, his Arabian Nights fantasy in Morocco.
Also worth a look Abercrombie & Kent