The most popular Vacation Rental sites in the US are:
VRBO = Vacation Rentals by Owner at www.VRBO.com
Book over 1 million listings across our family of brands. Perfect for Family Vacations, Reunions & Group Travel.
Another one that I use often, especially internationally, is:
Find Unique Listings in 30K Cities!Named to Top 10 Travel Websites – MSN Travel
24/7 Customer Service. Check it out.