There's a SIM900 based one around which allows you to configure which pins to use for Rx & Tx, useful when using a Mega. The cheap Seeedstudio one requires the use of pins 0 & 1 which messes up communicating with the board.
You may need to use SoftwareSerial, so check that it can be used on the pins the shield offers you. See The SoftwareSerial reference for further details.