uBlock Origin (adware blocking)
Pushbullet (send files and links to your mobile device)
Carat (text editor)
Chrome Remote Desktop (multi-platform remote desktop access)
Chromebook Recovery Utility (create recovery media)
Google Cast (send media to your TV)
Sprucemarks (auto-sort your bookmarks)
Stylish (apply your own custom CSS to any web site)
System
Windows Access (Samba / SMB / CIFS)
Fun
Drumbit (virtual drum machine)
Until AM (virtual DJ / mixing system)