Google Apps & Extension
Adblock Plus is the most popular ad blocker ever, and also supports websites by not blocking unobtrusive ads by default (configurable).
Boomerang allows you to schedule messages to be sent or returned at a later date. Write a message now, send it whenever, even if you're not online. Track messages to make sure you hear back, and schedule reminders right inside Gmail™
The "Docs PDF/PowerPoint Viewer" extension is now obsolete - we recommend trying Google's "Office Editing for Docs, Sheets & Slides" extension. By clicking on a link for a supported document format you'll be taken to the Google Docs Viewer. No more downloading large files and launching another application just to view it.
goo.gl url shortener is an extension which allows you to shorten the current website URL with the Google URL Shortener service http://goo.gl/
View and edit Microsoft Word, Excel, and PowerPoint files -- without needing Office installed on your computer.