By Bus/Coach
There are coach services from all nearby European countries - Slovenia, Italy, Austria and Germany, Czech Republic and Switzerland - into Croatia. You'll certainly be able to travel by bus to Zagreb - Zagreb Bus Terminal has a website on which you can check arrival timetables - although there are also direct international bus services to towns along the coast (Porec, Rovinj, Rijeka, Zadar, Split, Dubrovnik).

Travelling by coach is probably one of the cheapest ways of reaching Croatia, although perhaps not the quickest or the most comfortable! (Although we've had several emails praising Croatian bus services.)

Coach Travel In Europe
See the Eurolines website for ticket and timetable information for travel from your home country and Europe-wide.

Bus Companies/Bus Terminals
Take a look at the following websites for timetable details of routes into (and out of) Croatia:

AK Karlovac
Routes to Karlovac
APP
Variety of international routes
Autotrans
Rijeka-based bus company
Croatia Bus
Zagreb-based bus company, international services to Trieste, Graz, Vienna, Prague, Germany
Varazdin Bus Terminal
Shows timetables, routes to Austria
Zagreb Bus Terminal
Details bus departures from and arrivals in Zagreb. Very useful!

Autobusni Kolodvor is also a very useful website on which you can look up bus timetables for international routes to destinations in the whole of Croatia.