We are currently house sitting here in Kingston, Ontario for 4 months while the owners are away. They wanted somebody to live here and look after things until they get back. So we shovel the snow, and bring in the mail, and generally make the place look "lived in"...because it is being lived in!
It's not always totally "free". Usually, the longer term agreements have the house sitter paying the utility costs, as we are doing here in Kingston. Our total utility cost for the month of November was $260. But the shorter (one or two week) house sitting deals are typically in exchange for watering the plants, or looking after pets.
We never realized just how many people are looking for someone to live in their home. We signed up at www.housecarers.com when we realized how many opportunities exist worlwide.
There are opportunities, both short and long term, in France, Spain, Australia, New Zealand and many other places around the world, from 1 week to 6 months.
We plan on doing a lot of house sitting in the future!