Summary

I made a small rover bot with four-wheel drive and steer. For the environment detection I use three or four US sensor :

http://www.dx.com/p/hc-sr04-ultrasonic-sensor-distance-measuring-module-133696#.VZTfR_ntmko

The four small DC motor:
http://www.dx.com/p/tt-02-diy-car-model-tt-motor-encoder-w-wheel-black-yellow-257135#.VZTfe_ntmko

The software runs an Arduino Mega 2560.