Google hasn’t officially released their own repository to download the Google Chrome plug-ins. However, third-party sites such as Chrome Extensions have a large library of extensions and plugins ready for you to download. Many of the extensions they provide are pointless, however there few extensions that can make your Chrome browsing experience a bit more useful and easier.
  
Chrome Reader
A Google Reader extension that allows subscribing feeds directly from the Chrome address bar.

Google Translate
Simple extensions that allows the user to translate the current page using Google Translate.

Quick Bookmark
An extension to access the bookmarks quickly through popup.

Anti Ads
AntiADS is a simple and easy to use ads remover for Chromium.

Facebook for Chrome

An unofficial extension which lets you view your Facebook news feed and wall in Google Chrome.
You can also update your status by clicking on your profile picture.

Aviary Screen Capture
ake a screen capture of the visible portion of any web page and it will open in an editor where you can crop it, add text and arrow markup, get color information or move around areas. You can easily save it to your desktop or host it online. The extension also lets you quick launch any of Aviary’s web editor design apps.
May be these are worthless, but these are the extensions that I mostly use as a web surfer. You can suggest me more useful extensions in the comment box below.

Get Flash
Semi-universal extension for download flash and flash video.

Get YouTube
Download YouTube video in different formats: MP4 (1080p, 720p, 360p), FLV (HQ, Standard, LQ) and 3GP.