Why The Universe Is Probably Flat
A very interesting presentation describing why the Universe is thought to be flat. Another reason not mentioned is that in a flat Universe the total energy would be zero as gravity can have negative energy and negative energy of gravity balances out the positive energy of matter. This would allow a Universe to start with zero energy and then quantum fluctuations would produce the Universe.
Theoretical physicists have always believed that the Universe had to be flat and that is now appears to backed up by observations.