Onstar is a computer system built into certain luxury cars, with a built in hands-free cellphone feature. It would probably be extremely usefull in an emergency situation, but as of now, all ours is used for is an expensive toy. From the OnStar website:

What Is OnStar?

Driving directions, emergency assistance, up-to-the-minute stock quotes, email and more, all in your vehicle. OnStar offers a variety of services and three plans designed to help you through virtually any driving obstacle.

Air Bag Deployment:
Notification Signals OnStar if your air bags deploy
and contacts you to offer assistance.

Emergency Services:
We can locate you
and contact help.

Personal Concierge Services:
Plan entire trips, make reservations,
even get tickets to sold-out events.

Route Support:
We can guide you to
any destination.

Stolen Vehicle Tracking:
Advisors can locate a stolen vehicle
by satellite and contact the police.

Roadside Assistance:
Got a flat? Out of gas? We'll locate
your vehicle and send for help.

Ride Assist:
If unable to drive, we'll contact
a taxi or family member.

Remote Door Unlock:
We can send a signal to your car's
computer to unlock your doors.

Accident Assist:
After an accident, we'll guide you
through all the steps that follow.

Online Concierge:
Handle travel plans and
more through our website.

Remote Diagnostics:
We can run a diagnostic test of
your engine - while you drive.

Uses a five-million listing database
to provide location info.

The main problem with OnStar it that when you need to use it, you're voice isn't normal and the speech recognition has trouble understanding you. The following is a transcript of a conversation between me, my friend Chris, and OnStar.

Nyar: (turns Onstar on)
OnStar: OnStar Ready.
Chris: heh. cool. Get my entertainment!
Onstar: Dialing emergency roadside assistance.
Nyar: Stupid! (turns it off, then back on.)
Nyar: Get my entertainment (spoken slowly)
Onstar: Virtual Advisor Ready.
N: Music Reviews.
O: Pardon?
C: hee hee
N: Music Reviews.
O: Slower, please.
N: Mu-sic! Re-views!
O: Email it is!
C: (Laughing hard)
O: Pardon?Pardon?Slower, Please! Goodbye.
N: (turns it back on)
C: Get my bitches.
O: (plays orchestra-like sound) Which horoscope would you like?
N: Cancel.
O: Dialing emergency Roadside assistance.
N&C: errrr...
N: (turns it off and leaves, frustrated)