Bus services are excellent and relatively inexpensive. There are often a number of different companies handling each route, so prices can vary substantially. Luggage stowed in the baggage compartment under the bus costs extra (around 10KN a piece). Note that buses between Split and Dubrovnik pass through Bosnian territory so you'll need to keep your passport handy.
Autoherc Extensive services in Dalmatia and to Zagreb.
Autotrans Based in Rijeka but with services throughout most of the country; part of the Arriva group.
Brioni Pula Based in Pula. Connections throughout Istria and to Split, Zagreb, Vukovar, Trieste, Venice and Padua.
Clissa Only operates on the Zagreb–Trogir–Split route, but has electrical sockets, wi-fi, newspapers and water for passengers.
Croatia Bus An extensive network linking Zagreb to most of the country.
Čazmatrans (www.cazmatrans.hr) Buses from Zagreb to Slavonia and Dalmatia.
Samoborček Connects Zagreb with towns in Dalmatia.