I use hovclick (http://www.hovclick.com). I tried a few other software out there for this, but it was a bit to confusing (I'm a marketer.. not a coder). The editor is pretty easy to use, reminds me a lot of wordpress. Hovclick even works with google adwords to display the popup only if my keyword is within the url. The site looks pretty new though, as there are some pages missing. Popups work great though, i haven't had any issues, and the best part... they are unblockable. It's free too. Major plus.
This thread ranks high on google for 'unblockable pounder', so I'd like to place a mention that if you are looking for a pre-packaged solution that will allow you targeted placements of unblockable popups and popunders for a self hosted wordpress installation then I've a supported solution available at http://www.wptraffictools.com/video-demonstrations/popup-popunder-profiles/
most of the "unblockable" pop-ups shown here are not pop-ups in the true sense. They are simply DOM div-overlays which are made visible or displayable by user-events. A very quick way of achieving this sort of feat is to use the OVERLAY or PANEL class contained in the YUI(Yahoo User Interface) Library. This Library simply extends javascript class and functionality in a unique way. Specifically here, you use the show() or hide() methods of the PANEL class to show or hide the "pop-up". You can even style your overlay with custom css
Due to all the recent browser updates I've not found a script that supports perfect multibrowser capability, but I've seen it done on big sites. Any leads would be great (I'm not meaning popover overlays, but instead true popunders).