Poptin Email Grabber

You can set up a Free Popup Account that is free for the first 1000 site visitors each month, for a single domain.  More than that and it will cost $20/month for 10,000 visitors.  The free version is branded for poptin.com, but they offer a number of professionally designed pop-ups that can collect not only email addresses, but additional information using a custom form.  Besides sending you an email with the leads information, a nice dashboard lets you see reports and such.