View Like Us is a website that lets you preview sites in various resolutions, in a very convenient.
After you have finished the design of a site, you can preview how it looks on an iPhone, Wii browser, and various resolutions (from 800 x 600 all the way to 1920 x 1200).
The preview system is powered by AJAX, so switching between various resolutions is slick and easy (and requires no page loads).
It may not be an accurate representation (especially for the iPhone and Wii browser), but ViewLikeUs can help you get an idea of what needs to be tweaked. For example, I realized that lack of lateral margins in the design of Design Blurb (this blog!) makes it ugly on an iPhone (see image above) and low-res screens.
Do you use any other websites or apps to preview your site in lower resolutions? Recommend your favorites in comments.