I just came across a simple little tool called Responsive.is that is web based and allows you to enter any URL and preview the site from different device dimensions. Sure there are developer tools you can install to do the same thing or just resize the browser (as long as you’re not using IE)
Basically you enter any URL at the top and below you will see a live render or preview of that URL. On the right of the text box where you enter the URL you will see icons that represent the most popular responsive resolutions (desktop, tablet, phone, etc). When you select a resolution type the page below will react and resize to the appropriate height and width so you can see what each page would look like on those devices.
Here you can see what this site would look like as rendered by this tool:
I have found this to be very handy to use with clients who are not going to install development tools but still want to see what their site would look like on each device. Sometimes the simple tools are the best and this is one of them.