There are numerous great Add-ons available, so it's difficult to choose just a few good ones. These ones I've found to be fairly useful, making life a bit easier and increasing productivity.
Fireshot allows you to save screenshots of webpages to your computer. Useful for saving information for offline use.
While on a webpage, by clicking on the Fireshot icon on the top right of your browser, you can crop and edit the screenshot, then save it to your computer. By saving your preferences, you can set it to make the process much quicker, good for gathering information quickly.
It allows you to save the captures as pdfs or images, edit and annotate the images, email them, and upload them to sites such as Twitter, Picassa, Flickr and Image Shack.
Multirow Bookmarks Toolbar Plus
This Add-On allows you to hide your bookmark names by just displaying the icon, and to auto-hide the toolbar. It also for, as the name suggests, multiple rows of bookmarks, good for those who tend to collect a large number of websites.
Stylish allows you to 'restyle the web'; to choose new themes for webpages. It supports a number of major websites, such as Google, Facebook, Twitter, Tumblr and Reddit.
Web of Trust helps you to find trustworthy sites by screening website links and checking the safety of websites you might visit by putting a green circle next to the ones that are okay. Good when encountering suspicious, non descript links as well as for shopping and web browsing and greatly reduces the chance of getting viruses and malware on your computer.
Automatic Save Folder
Saves certain files in specific folders. You can set filters, specifying where to save different file types. For example, you could set jpg and png to the pictures folder, pdf and docx to documents, avi and mpg to videos and mp3 and aac to music. By setting the filters it cuts the time spent saving files to certain folders by doing these automatically.
Custom New Tab
Opens new tabs to a particular website or url. This add-on can be a solution for opening a new tab to the google homepage.
With this Add-On, by right clicking in a login field you can use a free login. Useful for sites you need a quick login to, or sites you may not use as much.