Zagreb has a well-developed and efficient public transport system that consists of trams, buses and trains. The tram network (route map ) operates 24 hours a day – from 4AM to 12AM there are 15 “day-time lines” (tram lines 1-9, 11-15 and 17), and from 12AM to 4AM there are 4 “night” lines (tram lines 31-34) which cover most of the day-time lines on a reduced (around 30-40 minute) frequency. Occasionally, buses replace trams on night lines. Tram line 3 does not operate Saturdays, Sundays and on public holidays. There are maps of the tram lines on almost every stop, so if you know the nearest stop to your destination you can simply figure out the shortest trip while you’re at the stop.
As for the buses, there are 113 day-time and 4 night lines. ZET buses cover the area outside the city center, as well as some neighboring towns that administratively belong to Zagreb county. The buses use the same ticketing system as the trams.
A historic funicular railway (uspinjača in Croatian), one of the steepest in the world, operates between the lower and upper towns. Monthly, daily as well as single tram/bus ticket can be used, otherwise there is a small fee for a ride.
If you are looking at value for money then you simply cannot go passed the ZET Value Card. This gem of a pocket sized card works via a top-up system, as soon as you add money to you card it is ready for use, a minimum amount of 10kn is needed to activate the card and the maximum amount peaks at 200kn. An added bonus is that by using this card a travel ride costs a mere 10kn, and that is the best price in town. The ZET value card can be used across all zones and lines and as an added bonus, it is transferable to other users. Where can you get your hands on this ZET Value Card? It can be purchased at all ZET shops and partner kiosks (Tisak). By purchasing at these places you can choose the amount you wish to credit your card with.
More info: http://www.zet.hr/vijesti/novosti.aspx