Modelica Tutorial Cover.jpg Introduction to Object-Oriented Modeling and Simulation with OpenModelica (PDF format, 6.6 MB) - a more recent tutorial (2012) by Peter Fritzson
Modelica Tutorial 1.4.jpg The Modelica Tutorial, version 1.4 (PDF format, 180 Kbyte) gives a comprehensive overview of all language elements with many practical examples. This version refers to Modelica version 1.4, but works still well as an introductory text.
Using Modelica for modeling physiological systems by Jiří Kofránek and Filip Ježek (PDF, 12 MB, in Russian) is a comprehensive tutorial with a focus on physiological modeling rather than general Modelica constructs.