Here is an open source robot with a TurtleBot 2 chassis that comes with a variety of sensors and tools to help you get started in robotics. The base comes with a docking IR sensor, cliff sensor, bump sensor, programmable LEDs, and various extension ports.
The YUJIN ROS Robot can be ordered with a 2200mAh or 4400mAh battery. It is open source and ready for your experiments.