Hello Flog and thanks for your post.
As the others have already mentioned, the most widely used and greatest 'viewed" website is likely to be the local MLS or Multiple Listing site for your geographic region. Unfortunately, however, listing any property to the MLS site is restricted to Realtors and those who can afford the membership to the site each year. In some cases, like here in Santa Clara County, California, the MLS is viewable by both the public as well as Realtors, which means that we have twice as much traffic as other sites. There are many Reatlors who will "list" your home on the local MLS for a small fee while allowing you to sell your home by yourself, and it's often worth it for the exposure to have this type of advertising to a large number of potential home buyers. To find these participants, check out FSBO.com.
Finally, I also like Craigslist for selling a home. It's a great resource, and as Dianne noted below, you need to constantly update your pictures, description or other information in order to keep the listing "fresh" on the list and to prevent it from dropping from the most viewed portion of the list.
Grace Morioka, SRES
Area Pro Realty
San Jose, CA