I do a lot of document signing....
Google Voice lets you make and receive phone calls!
Google Mail Checker lets you know when you've got Gmail
Google+ Notifications lets you know when stuff happens on G+
Share On Google Plus makes it easy to share the page you are looking at:
Forecastfox is an outstanding weather popup app
Chrome Remote Desktop is hands down THE killer Chrome app. It's what Chrome does better than any other platform
And of course, Pixlr Editor is great for editing pictures (like the ones in this post!)
Everything else I use was on the Chromebook when I got it I believe. If you have ANY questions about anything Chromebook related, please ask in the comments. I love being helpful