"the Java Universal Network/Graph Framework--is a software library that provides a common and extendible language for the modeling, analysis, and visualization of data that can be represented as a graph or network. It is written in Java, which allows JUNG-based applications to make use of the extensive built-in capabilities of the Java API, as well as those of other existing third-party Java libraries."
Jung is one of the most widely used graph/sna libraries. There is a fairly large and helpful community of users. JUNG's developers respond quickly to questions and comments.
JUNG is a library (with some nice examples) not an application, so it is not directly useful for doing sna. GUESS is one project that uses many of JUNG's features to build an application.
Joshua O'Madadhain, Danyel Fisher, Tom Nelson
Built in support for many aspects of Pajek net and GraphML formats.