Google Chrome

Auto-Reload
Reloads a web page on a regular basis. The interval is configurable through the "options" page.

Docs PDF/PowerPoint Viewer (by Google)
Automatically previews pdfs, powerpoint presentations, and other documents in Google Docs Viewer.

Forecastfox Weather
View current conditions, 7-day forecast, maps and severe weather alerts in an easily readable format.

Google Mail Checker
Displays the number of unread messages in your Google Mail inbox. You can also click the button to open your inbox.

Google Voice (by Google)
Make calls, send SMS, preview Inbox, and get notified of new messages. (US only)

Validity
Validity can be used to quickly validate your HTML documents from the address bar. The number of validation errors can be seen in the tool tip and the detail can be seen in the Javascript console.