Undeveloped or Vacant

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

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

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