1. Chromepad - This is a very useful extension which helps you take notes right from the comfort of your browser. So, if you find some interesting text on the web or maybe you just want to jot down a few words which came to your mind, Chromepad is the tool for you! The local storage box available from the Options menu of Chromepad lets you retrieve/view your older notes as well. Very useful.
2. Wikipedia - You may have probably guessed it. Wikipedia extension allows you to browse the mobile version of Wikipedia from its extension window. You dont need to visit the massive wikipedia page to access the articles anymore.
3. Aviary Screen Capture - This is more like a screenshot tool only with loads and loads of editing and uploading options. You can take screenshots of a particular webpage and edit it right from the comfort of your browser.
(Read another post about Google Chrome extensions : Useful Google Chrome extensions)
[Note : All extensions mentioned above also work with Chromium (the open source counterpart of Google Chrome). If you want to install Chromium, go through : Install Chromium Browser]