240px (screen width) x 320 (small phone)

320px (screen width) x 480 (iPhone)

480px (screen width) x 640 (small tablet)

768px (screen width) x 1024 (iPad - Portrait)

1024px (screen width) x 768 (iPad - Landscape)

Note: The content width may not be 100% perfect - 15px has been added to the iframes to estimate scrollbar widths
A tool by Matt Kersley - Fork it on Github