eValid™ -- Automated Web Quality Solution Browser-Based, Client-Side, Functional Testing & Validation, Load & Performance Tuning, Page Timing, Website Analysis, and Rich Internet Application Monitoring.

Explanations
Here are sample screenshots from a desktop host machine that show the starting page for Side-By-Side Comparison of Two Devices. That process results in two sub-windows being launched, after the UserAgentString for each has been modified to correspond to the device identified below the image.

In the sample screenshots shown below, both the original start page (left) and the page after clicking on the UserAgent link are shown (right).

The screens for the mobile devices are "larger" than in real-life because they are desktop renditions of the actual device's usable area. For example, the physical size of the Apple iPhone is 3.5 inches (diagonal) and displays 326 pixels per inch. This resolution is much larger than a typical PC monitor on which a typical resolution is about 72-96 pixels per inch. Accordingly, an iPhone appears on the desktop screen about 3X actual size.

Sample Dynamic Screenshots of SmartPhones (Open In New Windows)
Three Screen Setup With HomePage Displayed		Three Screen Setup with Agent String Displayed
Samsung I9100 Galaxy SII (HxW: 480 x 800) Apple iPhone (HxW: 480 x 320)
Sample Dynamic Screenshots of Tablet Devices (Open In New Windows)
Three Screen Setup With HomePage Displayed		Three Screen Setup with Agent String Displayed
Samsung Galaxy Tab 10.1 (HxW: 1280 x 800) Apple iPad2 (HxW: 1024 x 768)