Over the past few weeks, I read a couple of interesting articles on House Sitting including Sitting In Style by Belinda Archer in the Financial Times (requires subscription but you can use their 15 day trial).
I went looking for more info on the topic and found Sabbatical Homes which caters more to the Academic community.
The service is based in Manhattan Beach, California and defines itself as "a home exchange, house for rent, house swap, apartment for rent, off-campus housing and vacation home rental directory dedicated to academics worldwide".
They do accept non academics as well.
There is a set up fee of $25 (academics) and $45 (non-academics) to submit a house listing. The listing is good for 14 months. For home wanted, the cost is $0 (academics) and $10 (non-academics).
Read their Frequently Asked Questions for more details.
Back in May 2005, I wrote Want to take a longer vacation, try Digsville which offers similar services to a broader audience.