Diigo is a great service for anyone conducting research online. Essentially it's a highlighter and bookmark service, where you can bookmark pages, highlight specific passages, and add sticky notes to pages for later reference. These can be public notes to be shared among the community, or private ones. It's a great tool, and the recently released toolbar works very well.
One of our other favourite Chrome extensions is also made by Diigo, and it has the brilliant name: 'Awesome Screenshot: Capture & Annotate'. The name says it all really, as the extension will capture the visible portion of a web page, or the full web page, and then allow you to annotate it: adding text, drawing arrows and adding rectangles or circles highlighting particular portions of the page. It's a great tool for sending screen captures to people, and removes the necessity for an image editor to add annotation.
Following in a similar vain is Picnik, an extension for the online image editing service. It can also capture the visible portion of a page to be sent to Picnik, but clicking its icon will also show all the images on any given page, whereby you can send them to edited using Picnik. Another great little time saver.
This extension converts any foreign currency on any page to your local one instantly. Changes are highlighted in yellow upon loading the page, and the original number can be seen in parenthesis next to the adjusted one. This way it's easy to work out exactly how much Being British Tax we're being charged for any particular item, and adds context to things like financial results that are often quoted in USD.
There's nothing more annoying than clicking on a search result and finding that the piece of information you're actually interested in is buried at the bottom of the page, more often than not in some off topic forum posting. Google Quick Scroll deals with this problem, showing a menu in the bottom right that when clicked takes you to the part of the page relevant to your search.