I recently decided to create a popup on our home page to promote our monthly newsletter.
The industry seems to be divided on whether popups are a good idea. I suspect this is because they have been misused and abused by some sites. This has lead to the creation of popup blockers.
In our case we decided to use a popup (written using jQuery) but to restrict it (using a cookie) to make sure that it does not reappear for a site visitor for 20 days. In other words, if a new visitor came to billyfire.com today, and then visited every day for the next 20 days, the popup would not reappear until the 20th day.
I believe the value in the popup is to be able to relate specific information that you want a site visitor to focus on. If the visitor is not interested he can easily remove the popup and it won’t reappear (at least, not be 20 days).