Excellent little tool, very full featured, allows you to interact with the web page - and therefore log in to sites that require it - before taking your screenshot. Has multiple standard viewport sizes built-in, and allows you to enter custom ones, so you can take shots of webpages at any arbitrary size.
A very *big* con, however, is that pages that have a top section that take up the entire height of the browser window do not display correctly. Paparazzi seems to report the viewport height as being the height of the entire rendered page, so the screenshot comes out showing only the top section, stretched out to fill the entire page... This problem can be seen on this page:
I hope Paparazzi is still being developed, and that this will get fixed. It's a very promising tool.