Croatia is blessed with some pretty decent locally brewed beers. The brew you'll come across depends to an extent on which part of the country you are in, which is the local brewery and therefore the favoured local tipple. However, you'll find the leading Croatian brands Ožujsko and Karlovačko more or less everywhere.
How to order beer: ask for either a small or a large beer, or to be on the safe side, by the decilitre (“deci” for short). In Zagreb a small beer is 33 dl, a large beer is 50 dl (half a litre). However, on the coast, where beer warms up faster, your request for a small beer may result in a miniscule 25 dl, while 33 dl is considered “large”.
This is the range of beers you'll find in the average cafe or bar in the Zagreb area:
Ožujsko a light lager beer, slightly bitter and, as with most Croatian beers of this type, a not inconsiderable 5.2% alcohol. Probably the most widespread beer in Zagreb since it's brewed locally. Ožujsko Cool is the alcohol-free version.
Karlovačko another lager, probably no. 2 in Zagreb by sales and brewed in the neighbouring city of Karlovac. The alcohol-free version is Rally.
Pan a relative newcomer which hit the market in 1997, made in Koprivnica, northeast of Zagreb. Also available in a non-alcoholic version.
Tomislav a rich dark beer also made in the Zagreb Brewery, almost approaching a stout in character. 7.3% alcohol, it kicks like a donkey.
Velebitsko a favourite beer of younger generations, available in light, dark and mixed (rezano) versions. Said to be particularly fine in flavour as it's brewed using Velebit mountain spring water according to German purity laws. Not always easy to find in Zagreb but worth seeking out.
Medvedgrad a small Zagreb brewery purveying its ales through five of its own branded pubs. Choose between light and dark versions, mixed, extra strong and wheat beer (pšenično pivo).
Zlatni medo a microbrewery and pub on the corner of Savska and Vukovarska, offering light, dark and mixed beer.
I highly recommend Medvedgrad. The beer is excellent the indoor area looks real nice and the outdoor area is fabulous. You can find it at 49 on the main street which is the one that only trams run on and goes by one side of the main square.