A number of people have mentioned ultrasonic distance sensors, but our team has used IR distance sensors successfully in the past. The ones we've used are made by Sharp, and there are a variety of ranges to choose from. These are great for sensing game pieces anytime, or the walls of the field during autonomous. There are a number of vendors that sell the sensors, but I will refer you to the link that we generally use at Acroname Robotics:
http://www.acroname.com/robotics/inf...arp/sharp.html
One thing to watch out for with these guys is that they use a conductive plastic case. This can cause problems when the inspectors check to see if your cRIO is isolated from the robot chassis, so you will need to electrically insulate the Sharp IR sensor if you mount them on metal structural components.