On WPMayor.com we use Optin Monster (http://wpmayor.com/link/optinmonster). Formerly a plugin but now a SAAS, it can function on any website, not necessarily a WordPress one.

It's got tons of features including split testing and ready-made design templates. You'll see the best conversion rates when you offer a compelling proposition to site visitors. A PDF guide tends to work well.

To take it to the next level you can try content upgrades.

Syed Balkhi (CEO of Optin Monster) offers a great explanation on how this works:

"With Optin Monster you can also offer different Content upgrades are lead magnets (or opt-in bribes) that are created specifically for a particular blog post or page.

It replaces your generic eBook or “get free updates” offer with something much more targeted that strongly appeals to the person on that page."