WiseStamp allows you to create a dynamic email signature in no time at all. I get all I need with the free version.
Ever wanna tweet a great site you found, but ran out of 140 characters? goo.gl URL Shortener will shorten up your URL, giving you a lot more room in your tweet. This extension also offers a QR Code creator from the URL that you have in your browser.
Absolutely one of my favorite extensions! Ever had a zillion tabs open that you would like to save? TabCloud allows you to save the tabs that you have open. I find this particularly useful for when I am presenting. Simply open all the tabs you want for your presentation/class, save them with TabCloud and then when it comes time to present or for your class open up TabCloud and click on the tabs you want to appear and they will all populate!
Have a student that is distracted by all of the ads on a website? Install Evernote Clearly and eliminate all of the clutter on any given website, leaving just the content. This is a great extension!
Find a great article? Wanna save it for future use? Set up your Diigo account and save that article to a specific folder you have created using the Diigo extension.
I use this extension when teaching about privacy during our Digital Citizenship lessons. Behavioral tracking is big business and Collusion allows you to see what websites are tracking you and allows you to optionally block the invisible websites that are tracking you. Powerful extension to show students that the internet is not private at all.
The Ginger extension is a nice add on for your Gmail. This extension corrects your spelling and grammar.
Watch Doc notifies you when your Google Docs have changed.
I use it like the Diigo extension, but Evernote Web Clipper essentially lets you save anything you see on the web to your Evernote.
Docs Quickly allows you to quickly create a new Google Document, Presentation, Spreadsheet or Drawing.