Taxis

Taxis are relatively inexpensive and very convenient.  Always ask for the price of the taxi and rates from one place to the next beforehand. For example, one taxi driver may charge 35RM to take you from the Reclining Buddha Temple to Penang Hill, whereas another may charge as little as 25RM. Always bargain unless you're taking taxi in KL(because they charges by metre).

But, in Penang you don't really have to take a taxi - their public transportations are pretty convenient. You may only need taxis when you're going to your hotel or somewhere else from the airport.

Buses 

The Rapid bus (the public buses in penang) goes anywhere in your traveller map.

No 101 goes Batu Ferringghi

No 204 goes Penang Hill - it cost about RM2 or 3 to get there from Komtar bus terminal (a taxi for that same trip would cost about RM20 to 30).