1) AdBlock Plus
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This extension is pretty self explanatory. Blocks ads and stuff.

2) TinEye
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Find out where an image came from, how it's used, or find higher resolution versions!

3) AudioTheif Converter
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AudioTheif is the fastet youtube converter

4) Awesome Screenshot: Capture & Annotate
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Capture the whole page or any portion, annotate it with rectangles, circles, arrows, lines and text, blur sensitive info, one-click upload to share. Support PNG and shortcuts.

5) Elegant Calculator
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Pretty self explanatory.

6) Facebook for Google Chrome
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Extension which lets you read your Facebook news feed and wall. You can also post status updates!

7) Mail Checker Plus For Google Mail
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Displays the number of unread messages in your Gmail and Google Apps inbox. Preview mail, read, delete, archive and mark as spam!

8) Sexy Undo Close Tab
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Accidentally closed your tab? Tired of losing tabs in Incognito? Don't worry, everything is possible with this sexy extension!

9) Universal Search & IE8 Accelerators
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Universal Search is the missing search tool for chrome, includes searchbox with suggestions, highlight search and IE8 accelerators.

10) Turn Off the Lights
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The entire page will be fading to dark, so you can watch the video as if you were in the cinema.

That's all for now guys. If you guys like this I might add more 8)

EDIT: Added 10 more. Thanks to Ragnarock for helping me with some of these

1) PanicButton
Hide all your tabs at once with one single button and restore them later.
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2) YouTube quality selecter
Automatically plays YouTube videos with any available quality you like.
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3) Smooth Gestures
Mouse Gestures! Quicky and easily navigate webpages using the mouse or keyboard. Feel like a wizard.
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4) StayFocused
StayFocused increases your productivity by limiting the ammount of time that you can
spend on time-wasting websites.
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5) Facebook Photo Zoom
The best way to view photos on Facebook!
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6) Illimitux
Watch DivX movies in streaming. Remove limitation from streaming platforms
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7) Google Dictionary (by Google)
View definitions easily as you browse the web.
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8. Stop Autoplay for YouTube.
Stops YouTube autoplay but allows pre-buffering.
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9) Kick Ass(Asteroid Game)
Transform any website in an Asteroid game space!
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10) Jimmy Wales
Adds a nice Wikipedia donation banner to every single web page.
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^This one sounds stupid right?
1. Find a friend who uses google chrome.
2. Install this extension without their knowledge.
3. ???
4. Profit!