[Bryan Herbst]

Although it is not possible this year (and thus moderately unhelpful to you right now), the new control system has the capability for robot-to-robot communication.

So, you could get data from another robot (say GPS data, for example), and use that to navigate around the other robot. If you were really clever and had excellent programmers, you could also program in a sort of a map of the field too, and map out robots, obstacles, and such to make your autonomous mode even more robust and adaptable.

However, for the time being, you are pretty much stuck with IR, Ultrasonic, and Touch.

Someone mentioned sound- I personally don't think that this is a terribly great idea, as competitions tend to be quite noisy, and you would be picking up a lot of background noise, not to mention your own robots' noise.
One way you COULD make this work would be to detect very high frequencies (outside of any range you would get inside the field at the competition), and have two pre-made frequency emitters to give to your allies. However, I can't recall the legality of noise emitters at competitions, so make sure to check first.

EDIT: Something else I just thought of that would definitely not work in FIRST, but would be pretty cool- Dark Knight style "seeing" via cell phones, and mapping out where your bot is