Some people hate them. However they have proven to increase conversions from my experience.

If you are looking for a way to draw attention to a specific piece of blog news this Free WordPress Plugin will create a clean simple dhtml popup box easily and quickly on your WordPress blog.

You can see a sample at: Her Affiliate Manager

Like anything, the key is to not overuse the feature and this handy free wordpress plugin allows you to set how and when your visitors will see the box.

All of the settings you need in a wordpress pop up plugin are included. The only draw back, is that you are limited in the actual presentation/design. This free wordpress plugin creates simple basic html bloxes.

Worth checking out.