Part 1, Part 2, Part 3 are available on the respective hyperlinks. Here are three more you may find useful to use in your day to day teaching/admin:
Wanting to know exactly how many days it is until the summer holidays? Wish your life away with this extension - counts down/up to/from any date you choose. Also find out how many days you've been alive. Or how long since any date in history. I scrolled back to the 1500s until I got bored...
Quickly and easily create QR codes whilst browsing web pages. Great for creating an online treasure hunt around school perhaps?!
18. Bookmark My Tabs
You've got a whole load of tabs open and you wish to access them again in the future. This extension apparently allows you to avoid 'too many tabs open' syndrome (I didn't know this was a condition?). Useful to provide a host of pages to your class if they have the extension installed too.