Once upon a time i was looking for a printing platform for my printrbot simple after I had upgraded its print area. This lead me down the usual rabbit hole for a flat and available surface to print on.
Things tried:

1. Glass from an old computer scanner - I cracked three before giving up on them, they are tempered glass and will not be cut. After that they shattered from heat shocks.
2. Home Depot Glass Pains - These could be cut but cracked when shocked from heat.
3. Aluminum Sheets - This is a very popular option and I hear it works well. When i tried it i did not get the aluminum milled flat and it was too thin no to bend. I find that metals are too ductile to provide a flat surface. I know that a proper piece of aluminum is the favorite choice of many but with shipping the cost is too much.
4. Mirrors - I got two glass mirrors from reprap.me and loved them, they were the right size and have yet to crack. Best part though, they were $5! Cheaper than getting glass pains from the local hardware store. The only problem with mirrors are the thickness and they do not always come in the sizes I need.

This leads me to my current build plate, its a 200x300mm by 2mm thick carbon fiber board. Carbon fiber is very strong so it will hold its shape. When I ordered a board form ebay I did not know if it would be flat enough to print on. Upon examination the board is very very flat. It also has a nice shinny and glass like surface thanks to the resin that the fiber is bonded in. When i try to bend the board in my hand it flexes very little.

Other things to note about the carbon fiber, it was cheap from china. There are some defects in the layout of the fiber but that does not affect the smoothness of the board at all. I wouldn't go thinner than 2mm thick and if the print surface needs to be bigger I would recommend a 3mm thick sheet for a comfortable level of rigidity.

This is my first week with the carbon fiber bed so I will post any updates that will find!

UPDATE 2/4/2015:
No problems with the build plate. I use glue sticks as an adhesive. I get a lot less curling problems than with the glass beds I used. As long as there is glue down the print will stick and will be flat. I have sense printed a large object on the bed with no problems, 250mm long.