Taxi is most common service in Penang. The taxi driver can sent you to any places. However, taxis in Penang do not use meter, so you better ask the price before boarding the taxi. You can negotiate on the price quoted if you feel that the price is too high or wait for another taxi to come.
There are three different colour of taxi, red colour is most common, but the red colour taxi will be replace by brown Proton Exora very soon. The other taxi is blue colour taxi. Even the colour is different the taxi is provide same service.
Normally you can get a taxi at hotel, shopping mall, bus terminal, tourist destination or bus stop on road side.