MakeUseOf.com:
ViewLike.us is an online application that lets you quickly test website in different resolutions. You simply need to enter website URL into provided field and click on the “Submit” button. Once the page loads, you can click different resolution tabs to view how the site looks.
Currently the app allows to preview te websites in almost all popular resolutions: 320×396, 800x 600, 1024×768, 1152×864, 1400×900, 1600×1200 and 1900×1200.
You might also want to try BrowserShots, to test your site in different browsers.
Photo by ViewLike.us.