Google Voice

The Google Voice extension for Google Chrome adds a button to the toolbar, which displays the number of unread messages in your Google Voice inbox. It also gives you quick access to your most recent messages with transcripts, lets you initiate calls and send free text messages by just typing any number or contact name, and makes phone numbers on websites callable via Google Voice by just clicking on them.

It also adds a pop-up to any highlighted phone number that allows you to instantly initiate a call.

Requirements:

Google Chrome 4.0 or later