There is a service called Numbr (formerly Craigsnumber) that provides free disposable phone numbers. You say how long you need the number (one hour to one month) and tell it to what real phone number(s) you want it forwarded.

There are some additional advanced options and you can terminate or extend the number if desired. Get your number at the Numbr web site or call (415) 234 5678.

One suggested use for a disposable number is when posting a contact number on an auction or ad. I could also see using it for online transactions or prize registrations when a phone number is required. I'm sure there are lots of other uses. If you think of any, add them to the comments.

Source: inter alia