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Bus Croatia - all bus stations, times and schedules on one website. Also includes ferries.
Croatia Ferries - timetables and listings of all ferries within Croatia, and to all the various islands as well.
Trains in Croatia do usually NOT require reservations, except for IC Zagreb - Rijeka/Osijek/Cakovec, and ICN Zagreb to Split. The fees are about a Euro when made locally. The official site is HzNet, however it's in the local language and may be difficult at first to navigate - however, you can jump straight to the English version of the site.