Google has updated its Gmail calls service to support multiple lines, allowing users to manage several ongoing calls at the same time. Although Gmail calls have been supported for some time now, offering voice and video conversations as well as the ability to call cellphones, it's now possible to make or receive multiple calls simultaneously.
The new service isn't a party-line; instead, if a new call comes in while you're already talking to somebody, you can place the first person on hold and answer it. Similarly, if you're on a call you can put that on hold and dial a second person. Call waiting will flag up another incoming caller should you already be on the phone.
Voice, video and phone calls all support the new system, though only two outgoing calls to physical phones can be supported simultaneously. You'll need to have Google's voice and video plugin installed - which you'll already have if you've been making calls previously - but otherwise it shouldn't require any further setup.