#1: Page Ruler
Type: Developer Tools
Summary: Draw a ruler around an image to get the pixel dimensions and positioning on a page. This extension is great for anyone who develops websites, presentations, etc.
#2: Clipboard History
Summary: This extension allows you to retrieve items that you have previously copied. If you need to reference the item again, you no longer need to go back to the original source, simply pull up the extension and copy the information from there.
#3: Office Editing for Docs, Sheets & Slides
Summary: This extension adds an edit button when you open up a Word document, Excel Spreadsheet or PowerPoint in Drive. While you do have the ability to go to 'open with' and select Google documents to edit the file, I have found that many people don't know this trick. Adding the edit button just gives people more comfort in knowing that Office files can be edited when placed into Drive.
#4: Google Art Project
Summary: This extension simply pulls art work from the Art Project and displays them as your tab background. Nothing elaborate about it, but if you are into culture this might brighten your day. I love the Google Art Project. If you have never checked it out I suggest you drop everything and take a look. It allows you to walk through museums from around the world. You can even zoom into the paintings to see the brush strokes. It is amazing.