Contents:

1. For people who would like to compare pictures within their browser
2. "Time travel" tips, or how to start a video at a specific time in Youtube, Vimeo and Dailymotion
3. How to crop the black borders in a screenshot

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For people who would like to compare pictures within their browser

• Tile Tabs add-on for Firefox users;
• Tab-resize extension for Chrome.

Opera apparently has it as inner functionality. I didn't find anything about it for Safari nor for Internet Explorer.

In Firefox:

(Click...)