Hello, this is my first forum post! I'm in the process of building my first robot, and I'm not sure which motor shield to use. It will be a two wheeled autonomous robot that uses an Arduino Uno. I am using 2 1:120 geared motors. The robot will be similiar to the Bliss Robot (https://www.robotshop.com/letsmakerobots/node/28477), just with a different ultrasonic sensor. I now of the Instant Robot Shield made by PICAXE, but I'm not sure if this is the best option for my robot.
Thank You!
Which shield really depends on the current draw of your motors.
Some of the PICAXE boards have a spot for motor drivers on the circuit board.
You need to share a bit more relevant information. What board do you have, exactly. What motors are you considering using, exactly.
It was pointed out to me that I was asleep at the wheel when I typed that. 
I saw the mention of Uno, but, ignored it when I saw you mention PICAXE shield. The motor question still stands though.
For next time
http://www.dfrobot.com/index.php?route=product/product&filter_name=Romeo&product_id=656
I got one of these with a built in motor driver. It’s a great board, great price and arduino compatible. (I know you’ve already got one but this would work for next time)
Rugged Motor Driver
This was used on my 1st robot, and worked great!
Motors
Yes I’m using an Uno, which I already have. Maxhirez, how’s this one? http://www.ebay.com/itm/L298N-Dual-Stepper-Motor-Driver-Controller-Board-Module-5V-For-Robot-Smart-Car-/390504928126?pt=LH_DefaultDomain_0&hash=item5aebe8537e I like how small and simple it seems . And Birdmun these are the motors I ordered: http://www.ebay.com/itm/2-pcs-Smart-Car-Robot-Plastic-Tire-Wheel-DC-Gear-Motor-Robot-3-6V-/251194342261?_trksid=p5197.m1992&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D14%26meid%3D4648256998568618630%26pid%3D100015%26prg%3D1006%26rk%3D1%26sd%3D251194342261%26
With those motors
if you could find a shield that uses an L293, or, an SN754410, you would be fine. I don’t know this for sure, but, I have read more than once that the L298 bridge is rather more power hungry than its little brother the L293, and, similarly the SN754410. I mention the other two drivers, because, the motors you are using only pull 70mA @ no load. I would guess they won’t pull more than the rated limit for the L293 of 600mA, if they were stalled.
