Simple Web Application Prototyping: ProtoFluid
ProtoFluid is a free web-based tool which eases web application testing on various screen sizes and orientations.
It simply asks you you to mention the website to be tested and the desired screen size (sizes for popular devices are predefined).
Once launched, you can see the website inside a modal box with the mentioned size, and with a click, you can change the orientation.
For anyone, who prefers to complete this task on the browser-side, web developer add-on has the resize function, but keep in mind that you can still use Firebug or other tools at the fullscreen view with such a web-based app.
Also, it lets you design for CSS3 compliant (mobile) browsers using Media Queries.