For those who want to get into robotics but don't know where to start, let me point you in the right direction. I have been building robots from scratch for several years and have always been on the lookout for a good "kit" that would provide a sturdy robot frame that could be assembled without trouble and provide a good platform to test out my ideas. I had not been able to find such a kit until recently... SeeedStudio has started selling a kit called "MakeBlock", that includes several extruded aluminum pieces, bolts, geared motors, motor-hubs, wheels, tires, and a caster wheel so that you can easily assemble a small but sturdy (and reconfigurable) robot frame in less than an hour. The motors are geared down to reduce speed and have tons of torque for their size, also allowing them to be driven by a standard LM298 motor driver chip which is used on the Arduino Motor-shield sold on Sparkfun and at Radio Shack.
There are 2 basic kits available for a mobile robot, a 2-wheel drive version for $70 and a 4-wheel drive version for $120 - the 4wd kit comes with enough pieces to build 2 complete 2wd robot bases. The motors have a gearbox mounted to the front with a flatened ouput shaft to mate with the included motor/wheel hubs. The wheels are actually aluminum gears with fitted tires that can be removed. It even comes with battery holder and a neat little plexi-glass mounting plate with holes pre-drilled for the Arduino, so mounting that to the frame was incredibly easy.
With one of these kits, you need only add an Arduino and a motor-shield to get moving. While $70 might seem a bit steep, I was not disappointed with my purchase.