A few ideas to point out...
-Last year 1064 had an articulated tread system where they had 4 mini-treads in place of 4 normal tank drive wheels. The cool thing is, these treads could rotate. On one position all 4 treads could be flat on the ground, giving supreme traction. At another extreme, the treads could rotate at an angle so that only the two closest ends were touching. This would cut down on the turning friction (although I think the real reason was for the ability to drive over other robots).
Pictures here
Fundamentally, the design makes a lot of sense. As your wheels are closer together (wheel base is greater than the distance between the wheels), you would experience a lower turning friction. Unfortunately they didn't perform well at the regionals they attended...
-Team 16 had a 3-wheeled design in a somewhat triangular shape. The two wheels in the front were powered by drill motors, and then they had one rear wheel that could pivot, but it was powered by two CIM motors. This type of radial system does have advantages, because you have the ability for radial movements (circles, etc), and tank style.