Shows you how your customers will see your designs...
litmus is an online service for your to test what your website and email promotions will look like, before you finalize it. They give you screenshots for 13 different email clients, and 23! different web browsers.
Now it's not free, but it's reasonable. And it's worth the cost compared to the price you paid to have those websites and emails designed in the first place.
They do more then just previews too, like tracking, publishing, and version management.
Check it out Litmus