Undeveloped or Vacant

Land Watch
Site listing rural land in North and South America.
http://landwatch.com

LandFlip
Listings of lots and acreage for sale, land auctions and rural retreats.
http://www.landflip.com/

Land.net
Directory of residential and commercial land for sale or lease.
http://www.land.net/

Private Islands Online
Offers listings of islands for sale or rent in the US, Caribbean, Canada, Oceania, and Europe.
http://www.privateislandsonline.com/

Realtors Land Institute (RLI)
Organization of professionals specializing in land brokerage. Includes details on membership, property search, and an auction calendar.
http://www.rliland.com/

UK Land Investments Group
Develops parcels of undeveloped land from offices around the globe; includes investment opportunities, market statistics, and news.
http://uklandinvestments.com