Responsive design become very popular now. I have added some of the most popular online simulators, through this you can check how your site working in different screens.
1- Responsive Design Simulator
The responsive design simulator serves one purpose: to help designers, and audiences, visualize their responsive sites on popular devices. Simply type in the URL for your site and view the scrollable simulations below. Keep in mind, this is not a replacement for actual mobile testing as the mobile browsers can render CSS and HTML uniquely.
http://responsivedesignsimulator.com/
2- Responsinator
Responsinator helps website makers quickly get an indication of how their responsive site will look on the most popular devices. It does not precisely replicate how it will look, for accurate testing always test on the real devices.
http://www.responsinator.com/
3- Studiopress is anothor usefull responsive silumator
http://www.studiopress.com/responsive/
4- Cybercrab provides obest responsive site tester. Easy screen resolution tester.
http://cybercrab.com/screencheck/
5- Online tools for creative people. Test your website on any screen size including desktops, tablets .
6- PhotoFluid
ProtoFluid simplifies the development of fluid layouts, adaptive CSS and responsive design. It builds precise, dynamic viewports in which to test your work. This allows you to quickly effect changes and demonstrate benefits to interested parties.
http://app.protofluid.com/
Designs for User Experience 