Promo Instant Image Resizer - Automatically optimize images for web & social media (free!)
Responsinator - Designed to test responsive websites on different devices.
Responsive Image Breakpoint Generator - Turn one image into the ideal set of per-device images
Responsive Viewer - View multiple screen dimensions in one view
Placid - Use Placid to auto-generate images, videos & PDFs from reusable templates
Multi Screen Test - Test your site at different screen resolutions & devices.