The peer review system is in general single blinded: The reviewers will not be known by the authors of a paper by default. One reason for this is to guarantee that the reviewer can write his opinions on the paper without expecting any drawbacks whatsoever. However, I can imagine situations in which the reviewer may think about disclosing his identity:
The reviewer is not d'accord with the editor's decision. This happened to me once - as author: the reviewer wrote some kind of open letter to the the editor and put me and my coauthor in the cc. Basically, the reviewer complained that the editor weighted some other (unqualified) review high enough to not accept the paper.
The reviewer has ideas for further research based on the paper and thinks that a collaboration would be a very good idea. He could think about contacting the author directly but this would disclose his identity (in the case that the paper is not available as a preprint - which still happens sometimes). However, waiting until the paper would be published would be a waste of time.
Since I experienced 1. myself and I am thinking about 2., I would like to hear anwers to this question:
Under what circumstances (if any) should a reviewer disclose his identity to the authors of a paper?
Edit: A small clarification for point 2.: First, the submitted paper is not publicly available and hence, writing to the author would disclose the reviewers identity. Moreover, it is not about improving the actual paper (and wishing to become a coauthor) but about further work inspired by the paper.
In view of the current answer, I would also like to expand the current question a little bit (which is not worth a whole new question, I think):
... and what are arguments against disclosing the reviewers identity?