I made this site today as an easy way to share resolution-restricted pages with regular people (who don't have/understand browser plugins that let them resize to x by y). I often need to show someone what a particular page looks like at 1024x768 or 320x480, and sending a link that simulates that resolution seemed like a good solution.
Try http://www.simures.com/320x480/news.ycombinator.com
Current limitations:
-it's on app engine, so it needs a www.
-doesn't handle iframe busters yet