There have been many plugins published lastly aimed at helping bloggers add optin forms to their blogs. One was really cool but so overpriced it was out of reach of most bloggers.
Premium List Magnet is not only the best tool out there but also the cheapest. I have tested it extensively and I can’t recommend it enough.
It’s easy to use and it can create popover optin form, slide-in optin forms (from the top, bottom, left or right), sidebar optin forms, in-content optin form (integrated with yoru posts) you can select whether you want to display your optin form whenever a page is loaded, or once per session, or when visitor leaves (and many other settings).
But you can also use it just as an exit popup generator to display ads or to redirect to another web page. You can also use it as an advertising tool to show some banners…
And the templates are really premium ones (their graphic designer is simply awesome).
You can check how I use it at http://WsoXFactor.com