What good robot shield are out there for arduino?

What robot shield's for arduino do you use and why did you choose it?

For robotics, to facilitate interfacing to servos and sensors like Ping sonars, look for
something with a bunch of standard 3-row male headers [signal-power-gnd]. They
are 10X easier to connect than regular Arduino boards.

Picaxe makes a robot shield that is compatible with Arduino: Instant Robot Shield - Hardware - PICAXE

I used a Dagu Red Back Spider controller (based on the Arduino Mega) in my first robot.
http://robosavvy.com/store/product_info.php/manufacturers_id/35/products_id/1574

Then I started designing my own boards with 3A switch mode regulators. I use 2S/3S Lipo batteries to power my robots. The 3A regulator drops the voltage to 5V. This way I can plug in lots of servos and sensors on my board.
My board based on the ATmega328 http://www.bajdi.com/bajduino/bajduino-3a-3/
And my latest board based on the ATmega1284: http://www.bajdi.com/something-different-a-crash-robot/bajduino-mega-3a-2/

I like to use any motor shield. You do not really need anything to interface with sensors, but motors in most cases require either a controller, or a transistor+cap(if you want to risk frying your arduino)

I would go for the motor shield that seems to suit you the most.