The RTL-SDR uses the RTL2832U chip as its ADC and USB interface processing chip. It also has 8 GPIO (General Purpose IO) ports available which are by default unused by the original DVB-T dongle application. However, which the right modifications to the SDR drivers, these GPIO ports can be activated and potentially used for applications such as antenna, filter, pre-amplifier and attenuator switching.
Over on his web site S57UUU has been experimenting with these GPIO ports and has put up a short tutorial/set of notes on how to connect to the ports and how to modify the RTL-SDR drivers to set the state of each pin. You will need basic programming and compilation knowledge to understand how to activate these pins in the drivers, as well as good surface mount soldering skills to be able to connect wires to the pins.