Since 2010, Google Voice has provided Gmail users the ability to make free telephone calls to anyone in the U.S. and Canada through their Google Talk chat client - right in the browser. If you're not taking advantage of it, you should be.
In 2007, Google acquired an outstanding product called GrandCentral, and thereby created Google Voice (GV). GV is a telephone management service, a free one at that, which assigns a user a local telephone number, and allows the user to assign that number to ring their cell, office, home, and any number of other extensions. It also is a self-contained voicemail service, voicemail transcriber, call recorder, and more.
When Google Talk made phone calls possible through the browser, they allowed Google Voice users to attached their phone number to their Google Talk account, making phone calls inbound to Google Talk possible, and attaching the user's GV number to the Caller ID on outbound calls. All of a sudden, business calls from the computer became simple - and no hiding of Caller ID was required. But carrying your laptop (and more so, your desktop) around town for the purposes of making a phone call is a bit cumbersome - so users were left using their cell phones (perish the thought) through GV's servers to make outbound calls.