Recently I was searching for a decent plugin that works well and looks great with the capability to attract email newsletter subscribers. My search ended with finding a WordPress premium plugin called Ninja Popups. The plugin seems to work smoothly without any much load on your blog installation and does the chore neatly.
The neatly styled plugin interface has several options to configure your popups (similar to a light box) when a visitors reaches your page. The content of the light box popup could be a message, a Facebook like box, a subscription form or even a video. There are several templates and options that the plugin works on which you can see on their plugin page (Ninja Popups).
If you have the time to check out this plugin, the website has a demo of all the different modes the plugin operates in, let me know your feedback on using the plugin. Just thought of recommending it because, it is important to retain your customers who reach your blog as you should not always bank on the Search Engine traffic – build your own circle of trusted followers!