Okay, for the newest MacBook, there's a great way to eject the disc with no problem!

I had tried everything from Terminal commands to credit cards and this one is really easy.

Restart the Mac.
At the startup/login screen:
Tilt at about 45 degrees towards you. (You should be sitting in front of the laptop)
Press the eject button multiple times.
Wait about 30-45 seconds.
If the disc does not eject, press the eject button multiple times again and the disc should eject after a while.

The trick is to be patient.
If this doesn't work, do the same method while holding down the trackpad (Newest Mac: Lower right corner)

Hope this works!

I just wanted to give my thumbs up to this comment -- I had a stubborn DVD in my 2009 intel MBP and this was the only solution that worked!

Fantastic, thanks, this was bugging me all day!