This is a pretty nice little app from IconFactory that I think would be a great port:
xScope is six tools in one that will help any designer do their job faster and produce more accurate results:
• Rulers - On-screen rulers that measure distances and angles
• Screens - A dynamic view of smaller screen sizes and web browser content areas
• Loupe - A magnifying glass that gives you a close-up view of your work
• Guides - Markers for precise horizontal and vertical alignment
• Frames - Markers for precise width, height & origin alignment
• Crosshair - Finds the exact location on screen
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