The paper The Polyadic pi-Calculus: a Tutorial (1991) by Robin Milner doesn't appear to have been mentioned on the front page previously of LtU before. I think it is worth pointing to for beginners, since it appears to do a good job of covering the basics of pi-Calculus.

Abstract: The pi-calculus is a model of concurrent computation based upon the notion of naming . It is first presented in its simplest and original form, with the help of several illustrative applications. Then it is generalized from monadic to polyadic form. Semantics is done in terms of both a reduction system and a version of labelled transitions called commitment ; the known algebraic axiomatization of strong bisimilarity is given in the new setting, and so also is a characterization in modal logic....

For those fairly new to the group, pi-calculus is a formal calculus for studying concurrent computation. This would be a good thread for others not only to comment on this paper, but to mention other good entry points for the study of concurrency, and other calculi related to the pi-calculus.